WHEEL LOADERS

Reference: Service Manual Modules; "Brake Systems Operation, Testing, and Adjusting", "Power Train Disassembly and Assembly", "Power Train Specifications", and "Operation and Maintenance Manual" for machines listed.

Problem:

Repetitive high energy service brake usage may result in overheating with rapid oil degradation, causing poor brake and axle component life. Overheating is frequently not recognized by an operator since oil disc brakes do not heat fade.

Brake overheating can be defined as brake operating temperature conditions which will produce failures or undesirable side effects. In the case of four piece axles with enclosed oil disc brakes, overheating has occurred if:

1.The axle power train oil is burnt. This is often the first noticeable evidence of brake overheating and is the result of extended periods of operation with the oil above 120 degree C (248 degree F).

2.The brakes may begin to chatter. This occurs after the oil is burnt to the point of friction modifier depletion causing a change in frictional characteristics of the brakes. Chatter is a rapid stick-slip phenomenon which sets up a resonant vibration usually just before rotation ceases. Do not confuse this with new disc resonance. Study the condition of the oil and oil operating temperature to determine if the chatter is produced by brake overheating.

3.Brake piston seals become hard and/or the seal backup ring becomes deformed, allowing brake system oil to leak into the axle. Seal damage by overheating should not be confused with seal damage caused by incorrect assembly, contaminated oil, or rough surfaces. Seal damage from overheating is the result of extended periods of temperature in excess of 250 degree C (475 degree F). Temperature seen at the piston seals is the result of excessive piston temperature which is not the same as sump oil temperature.

4.The friction material on the brake discs become glazed, brittle, or black in color. In severe cases, the friction material may even separate from the disc in large pieces giving the disc an eroded appearance. Friction material damage indicates disc surface temperatures above 260 degree C (500 degree F). Color by itself can not be used to determine damage. Heat damaged friction material can be identified by the relative ease the fibers can be scraped from the disc surface. Compare by scraping the friction material of a new, oil soaked disc. Glazing can be identified by a shiny appearance. The normal surface has a porous appearance. Do not confuse overheating damage with handling damage, water damage, or contaminated oil damage. Note that glazing is also caused by using gear oil with EP additives. (See LUBRICATION page 8)

5.The brake piston and brake reaction plate have dark areas caused by excessive temperature.This is surface annealing. In severe cases, the piston and reaction plate may even have areas of plastic deformation or material smudging. The piston and reaction plate may be discolored and may be warped. Piston and reaction overheating damage indicates localized temperatures for extended period of time in excess of 260 degree C (500 degree F). Do not confuse overheating damage with oil contamination damage or resultant damage when brakes are allowed to go to total wear out.

Solution:

There are many possible reasons for brake overheating with an equal number of solutions. The following information is a step by step guide through the most common solutions. This information will help determine if an attachment brake oil cooler is required for your customer's application.

Brake overheating and related problems can also be caused by poor maintenance and incorrect service. Proper maintenance and service is the key to reduced usage cost. The enclosed information includes recommended maintenance and prevention methods for the most common service errors.

Oil Temperature:

Axle sump oil temperature is the measurement that will provide the best information to determine the magnitude and solution of a problem.

Temperature strips and heat guns are very useful for trouble shooting by comparing one brake to another, however, they do not provide direct comparison to axle sump oil temperature because of the effects from ambient temperatures, engine heat, and housing densities. Care should be taken when deciding where to place a temperature strip or what surface to shoot with a gun. Unless you are comparing one brake to another, the normal higher temperature at the brake area may cause unnecessary concern. When oil sump temperature is required, the axle housing oil fill plug should be slowly removed and a calibrated temperature probe placed directly into the oil. Hold the probe in the oil until the temperature reading stabilizes.

With some applications and operating errors, the sump oil temperature may never maximize during a full shift operation. To minimize this effect, take measurements towards the end of the shift after at least six hours of operation. To improve consistency for measurements taken on different days, take each measurement after the same hours of machine operation.

Use the temperature chart on page 17 to determine overheating severity.

Mechanical Conditions:

A.There is brake overheating in both front and rear axle.

NOTE: On newer machines with full hydraulic brake systems, the brake pedal valve is biased to activate the rear brakes first. Approximately 345 kPa (50 psi) will be sent to the rear axle brakes before the front brakes see pressure. This pressure difference will remain until brake release. Under normal conditions, this biased pressure difference will cause the rear axle sump temperature to be 3 to 11 degrees C (5 to 20 degrees F) hotter then the front axle.

1.Excessive residual brake pressure is present after brake release. Earlier air/hydraulic brake system machines should have no residual pressure. Newer hydraulic/hydraulic brake system machines should only see normal hydraulic tank pressure 35 to 70 kPa (5 to 10 psi) at full release. If mechanical conditions are not correct or there is operator error, the rear axle will see the effects first, causing sump oil temperature to be as much as 28 degrees C (50 degrees F) hotter than the front axle.

a.Foreign material is under the brake pedal heel not permitting full brake release. Keep cab floor clean of all dirt and foreign material. Cut added mats back away from pedals. Cut 19 mm (.75 in) off pedal heel. New machines have shorter pedals that do not have to be cut.

b.Brake pedal pivot pin is binding or the pedal valve spool is sticking. Check pedal base and valve for contamination and corrosion. Repair or replace as needed.

c.Brake pedal valve drain line on newer full hydraulic brake system machines is restricted or there is excessive back pressure not permitting normal brake pressure release. Repair any line restrictions. If back pressure is still present, contact the appropriate Product Analyst for your machine to consider a dedicated drain line.

2.Transmission neutralizer is not functioning, is not properly adjusted, or neutralizer lockout switch is activated. (See APPLICATIONS AND OPERATING ERRORS page 6)

3.The brake discs are severely glazed or severely worn, diminishing braking capability. This results in higher braking pressures to achieve stopping ability, causing overheating. Inspect the brakes for glazing and wear. (See MAINTENANCE AND SERVICING page 9)

B.Brake overheating at one axle only.

1.Excessive residual brake pressure is present at one brake, or both brakes on one axle, after brake release. Earlier air/hydraulic brake system machines should have no residual pressure. Newer full hydraulic brake system machines should only see normal hydraulic tank pressure of 35 to 70 kPa (5 to 10 psi) at release.

a.There is a restriction in the brake lines between the pedal valve and the brakes. Inspect the lines for an external restriction or internal foreign material. Correct or replace as needed.

If the machine has full hydraulic brake system and excessive residual pressure or excessive temperature is at the rear axle:

b.Foreign material is under the brake pedal heel holding the biased pressure to the rear axle brakes. Keep cab floor clean of all dirt and foreign material. Cut added mats back away from pedals. Cut 19 mm (.75 in) off the pedal heel. New machines have a shorter pedal.

c.Brake pedal pivot pin is binding or the pedal valve spool is sticking, holding the biased pressure to the rear axle. Check pedal base and valve for contamination and corrosion.

d.Transmission neutralizer is not functioning, is not properly adjusted, or neutralizer lockout switch is activated. (See APPLICATIONS AND OPERATING ERRORS on this page)

2.Brake piston is not retracting at brake release. For machines equipped with piston retraction springs and two piece seal assemblies, the piston should retract 0.5 mm (.02 in) immediately after brake release. (See MAINTENANCE AND SERVICING page 9)

a.Piston seals are damaged or were incorrectly assembled, preventing normal rapid retraction. Replace the piston seals, check for leaks, and check piston retraction.

b.Piston retraction springs are damaged from extended use at temperature in excess of 260 degree C (500 degree F), or were left out at last assembly. Replace springs. Check piston retraction before releasing the machine.

3.The brake discs at one axle are severely glazed or severely worn, diminishing braking capability, causing overheating at the opposite axle brakes. Inspect the brakes for glazing and wear. (See MAINTENANCE AND SERVICING page 9)

Applications And Operating Errors:

A.Brake overheating during high brake duty cycle applications such as trench back fill, truck loading, hopper loading, and log sorting.

1.Depending on machine configuration, the operator has prevented transmission neutralizing by only using the right brake pedal or has pressed the top of the transmission neutralizer lockout switch. The transmission neutralizer is not functioning. Transmission neutralizes too late with excessive pressure at the brakes. Inspect the neutralizer function. Repair and adjust as needed. Dependent on machine configuration, instruct the operator to only use the right brake pedal or only depress the top of the neutralizer lockout switch during steep slope operation. Follow-up as required.

Neutralizing settings on machines shipped from the factory are acceptable for most applications, however, they may need adjustment dependent on application and operator preference. For example, the transmission may neutralize too early when working continuously on a slope, causing the machine to jerk. The transmission may neutralize too late when working level ground, causing the brakes to feel harsh. Earlier machines neutralize at a set air pressure. Newer machines have an adjustable switch located at the left brake pedal base. In the appropriate Brake System Specification Service Manual Module for the machines listed, a typical dimension of 6mm (.24 in) brake pedal travel is used for adjustment. If brake pedal travel is increased, the transmission will neutralize later with more pressure at the brake. For best results when addressing brake overheating, the neutralizer switch should be adjusted so the transmission neutralizes early with minimal brake pressure. For the machines listed operating in high brake duty cycle applications, a recommended brake pressure, when transmission neutralizes, would be 345 to 480 kPa (50 to 70 psi) pressure to the rear axle brakes.

NOTE: Neutralizing the transmission permits higher engine speed for better hydraulic response while preventing brake drive through. Because an operator is heavy on the brake when the transmission neutralizer is not used or the transmission neutralizes too late, even repetitive steering with the machine in no or slow motion increases brake overheating. During productive high brake duty cycle applications, tests have shown 30% less pressure on the brakes when the transmission neutralizes can reduce the braking energy 50%.

2.Operator is using the brakes during machine acceleration. Monitor brake pressure and drive shaft speed during operations. Watch machine brake lights during operations. Check for correct use and correct setting of transmission neutralizer. Instruct the operator and provide training as required. Follow-up as required.

3.Operator is resting a foot on the brake pedal while operating the machine. Watch machine brake lights during operations, or monitor brake pressure and drive shaft speed. Instruct the operator and provide training as required. Follow-up as required.

B.Brake overheating during repetitive load and carry applications or frequent travel.

1.Operator is relying entirely on the service brakes to slow the machine down from high travel speeds. Special operating techniques are required when taking advantage of the increased travel speeds available using the optional ride control. As the machine approaches it's travel destination, use the engine to reduce speed before applying the brakes. Provide operator training and follow-up as required.

Some users may wish to disable 4th gear, or both 3rd and 4th gears, to limit the top speed of the machine. This can be done by making modifications to the existing machine harness. Reference Service Magazine May 25, 1998 page 41 for 910E through 928F including IT12B through IT28F. Reference Service Magazine February 23, 1998 page 10 for 928G and 938G, including IT28G and IT38G. Reference Service Magazine August 8, 1994 page 5 for 936F through 970F.

NOTE: Brake energy and heat generation is four times greater at 35 km/h (22 mph) than at 21 km/h (13 mph). Deceleration with the engine will slightly increase cycle time over using the brakes alone. However, the increase in production over a machine not equipped with ride control is still substantial.

2.Operator is using the service brakes for machine retarding to control travel speed down slopes. Use manual shift selecting appropriate gear to provide engine retarding. Provide operator training and follow-up as required.

Some users may wish to disable 4th gear, or both 3rd and 4th gears, to limit the top speed of the machine. This can be done by making modifications to the existing machine harness. See item 1 for reference material.

NOTE: Repetitive retarding with the service brakes can produce continuous heat generation. Axle oil sump temperature may never stabilize. This is an operating error that can only be corrected with engine retarding. An attachment brake oil cooler will not resolve the problem.

3.Operator is resting a foot on the brake pedal while operating the machine. Watch machine brake lights during operations, or monitor brake pressure and drive shaft speed. Instruct the operator, provide training as required, and follow-up as required.

4.Carry loads, machine attachments, and trailer loads exceed the recommended for which the machine systems were designed. Reduce the carry loads to recommended. Equip trailers with their own braking system that works in tandem with loader braking system.

Lubrication:

1.In the axles for the machine listed use only recommended oil type and classification Transmission/Drive Train Oil (TDTO) oils that comply with the TO-4 specifications. Do not use gear oils. Some gear oils contain EP additives. EP additives can place a high glaze varnish like finish on the friction disc under heavy braking and elevated oil temperatures. Glazing diminishes braking capability resulting in needed higher brake system pressures to achieve same stopping ability causing brake overheating.

2.The factory fill axle oil for the machines listed is Caterpillar TDTO, TO-4 SAE 30W. This is the recommended oil to use during ambient temperatures above -25 degree C (-13 degree F). For elevated axle oil temperatures two alternative oils may be considered.

a. When not subject to cold start-ups below -15 degree C (5 degree F), the use of TO-4 SAE 50W may be considered. At elevated oil temperatures, 50W will provide better oil film thickness protection for gearing and bearings. 50W oil may reduce axle oil temperature in high speed load and carry or traveling applications. 50W will not reduce oil temperatures produced by high energy braking, will not extend brake discs or brake piston seal life, and will not prevent oil degradation (burnt oil) problems.

NOTE: Until the axle oil reaches operating temperature, the use of TO-4 50W in the machines listed may cause some loss of performance and a decrease in fuel economy. For this reason, 50W is not recommended for 910E through 938G Wheel Loaders, and IT12B through IT38G Integrated Toolcarriers except for known high ambient temperatures and continuous applications producing known elevated axle oil temperatures.

b.Caterpillar now offers Transmission/Drive Train Oil (Transmission Multi Season), TDTO (TMS) partial synthetic blend that meets the Catrerpillar TO-4M specification. TDTO(TMS) is an option to replace TDTO SAE 10W and 30W in certain applications. This oil will function at lower ambient temperatures preventing a loss of performance and decreased fuel economy, yet has a stable shear formulation providing SAE 30W oil film protection. Being a partial synthetic blend, it will not oxidize as rapidly. This can extend the useful life of the oil at elevated temperatures and enable SOS sampling because oil will not degrade to black color as rapidly. TMS oil will not reduce oil temperatures and will not extend brake discs or brake piston seal life.

NOTE: Use only multi grade oils that comply with the Catrerpillar TO-4M specifications. Multi-grade oils that do not meet the TO-4M shear-stability requirements do not provide sufficient high temperature oil film thickness.

3.Monitor the axle oil for water contamination. Check for axle housing oil leaks. A leak can also allow water to enter. Water contamination breaks down the bond between friction material and disc. Friction material will erode and separate from the disc.

4.If brake chatter is the result of burnt oil depleting the friction modifiers within the oil, the use of friction modifier additives may only hide a more serious problem. Study the condition of the oil and oil operating temperature to determine if the chatter is normal break in or brake overheating.

5.If brake chatter is the result of burnt oil depleting the friction modifiers within the oil, the use of friction modifier additives may only hide a more serious problem. Study the condition of the oil and oil operating temperature to determine if the chatter is normal break in or brake overheating.

Maintenance and Servicing:

Preventative maintenance can extend brake and axle component life. Scheduled servicing can prevent resultant damage, greatly reducing service cost and labor intensive repairs. Correct repair methods can prevent premature failures.

1.Change axle oil as soon as burnt oil is detected. This may be determined by review of the oil color, oil smell, SOS oil condition analysis, and developing audible brake chatter. Use SOS analysis for wear contamination and oil condition to help determine oil change interval. See the temperature chart on page 17 for recommended oil change intervals depending on axle sump oil temperature.

2.Inspect the friction disc thickness at each oil change to determine when the brakes need servicing. Planning for brake servicing before a total wear out will prevent resultant damage from high iron contamination, labor intensive repair, and unscheduled down time.

To inspect for disc wear, there are inspection ports (one for each brake) along each axle center housing assembly. These can be located on the front or rear side of the axle housing depending on machine configuration. Some of the inspection ports lie below oil level. It is recommended to perform the inspection at the oil change interval after all of the oil is drained, or drain the oil below this level prior to inspection. Some of the inspection ports at the rear axle lie above the trunnion support and some lie below the support. Oscillation of the rear axle may provide better access to the inspection ports. See illustration.

Typical location of Inspection ports

The most important measurement is the depth of the oil grooves. The grooves allow oil flow across the friction surfaces providing both lubrication and cooling. As the oil groove depth decreases, the friction material will wear more rapidly. For this reason, it is important to replace the disc as soon as possible once the oil groove depth has reached the maximum wear limit 0.32 mm (.013 in).

The best way to check oil groove depth is with wire gauges. Check the oil groove depth on both sides of each brake disc in both front and rear axles. This is necessary because wear may vary from one axle to another, from one side to the other, and even one side of the disc to the other side. Significant wear at one brake compared to the other brakes may indicate a mechanical problem at that brake. The following charts show the estimated wear remaining for a given groove depth.

Disc Thickness Wear Chart For 910E Through 924F Wheel Loaders; IT12B through IT24F Integrated Toolcarriers.

Disc Thickness Wear Chart For 936F Through 970F Wheel Loaders; IT38F Through IT62G Integrated Toolcarriers.

Disc Thickness Wear Chart For 928F Through 928G Wheel Loaders; IT28F through IT28G Integrated Toolcarriers.

Brake disc wear for the 950G Wheel Loader and the IT62G Integrated Toolcarrier can also be measured with the Service Brake Wear Indicator. Each individual service brake uses an indicator pin to help determine the amount of wear on an existing brake disc. An indicator pin is located on each side of the front and rear axle center housing. See illustration.

Area "B" shows indicator pin location.

The amount of disc wear is determined by subtracting the measurement made at the indicator pin from the new disc baseline measurement. The new disk baseline measurement is recorded electronically on the transmission ECM. Reference the Service Manual Module, Testing and Adjusting, SENR1387 for procedure instructions. The chart below provides an estimate on the amount of service life remaining. A disc wear amount of 1.46 mm (.058 in) indicates that the disc needs to be replaced as soon as possible.

Disc Wear Chart For 950G Wheel Loader And IT62G Integrated Toolcarrier When Using The Service Brake Wear Indicator.

NOTE: If you install a new brake disc in any of the four service brakes, you must enter a new baseline value electronically on the transmission ECM.

3.If the friction discs are allowed to go to a total wear out, metal to metal contact (spalling) between the disc, the piston, and reaction plate will occur. When this occurs, the complete axle arrangement must be thoroughly cleaned. The axle center housing and differential group must be thoroughly cleaned with a high pressure wash. Both final drive planet carriers must be removed from each axle housing leg and the axle housings cleaned with a high pressure wash. If the cleaning process is not done correctly, the debris will continue to contaminate the oil causing the brake friction disc to wear more quickly, producing a premature brake failure.

NOTE: When using a high pressure wash to clean an axle arrangement, dry all internal components and surfaces with dry compressed air immediately after washing. To prevent oxidation, apply a film of oil to all internal components and surfaces using the correct power train oil for your machine.

4..When cleaning debris from the brake friction disc use only chlorinated solvents, diesel fuel, kerosene, and petroleum distillates such as Stoddard Solvent or naphtha. The friction material is porous. Incorrect cleaning materials or processes can damage the friction material and cause premature failure. Follow these precautions.

-Do not use petroleum solvents hotter than 38 degrees C (100 degrees F).

-Minimize the time solvents contact the discs.

-Use towels or dry compressed air to dry the discs after washing.

-Do not stack the discs after cleaning.

-Before installing the discs into the axle, oil soak the discs with the power train oil to be used.

For extended storage, hang the disc vertically. Protect with plastic sheet covering to prevent moisture absorption and dust accumulation.

5.When servicing the brakes, remove the piston keeping the piston aligned with the housing. Prying on the piston while not maintaining alignment can damage both piston and housing. Replace both seals on each brake unit being serviced. If the machine has high service hours or brake friction disc needs to be replaced also, replace all piston retraction springs for that brake unit. Before installing the discs into the axle, oil soak the discs with the power train oil to be used. Use the following procedure to replace the brake piston seals.

Correct Installation of Brake Piston Seals.

-Do not leave the seal packages open before use to prevent exposure to moisture and dirt.

Don't use any seal assembly where the backup ring is crimped from shipping or mishandling.

-Thoroughly clean the seal grooves in the center housing assembly or piston. Inspect the seal grooves and sealing surfaces for any marks or damage that may cause a leak. Replace any damaged components if needed.

-On machines using a seal assembly with a backup ring, check the fit of the backup ring to the piston before installing into the center housing. If the backup ring is too tight, it will not slide loosely over the piston as it should. To increase the diameter of the backup ring, warm the backup ring on a heated surface or a dry air oven at a temperature of 40 to 60 degrees C (104 to 140 degrees F) for approximately five minutes. The backup ring should now fit loosely over the piston bore. If it is still too tight, return the seal assembly for another.

-Install the seals into the center housing or piston, dry. Install the seals for correct orientation. For machines using the seal assembly with a backup ring, install the rubber portion first with the angled side opposite the pressure side of the housing. Make sure the seal is not twisted. Next install the backup ring with the rounded inside corner facing towards the rubber seal. The side of the backup ring with the rounded inside corner is identified with a color mark. The color mark should face towards the rubber seal. Make sure the backup ring is not twisted. For best results, do not coat the seals with oil. Lightly coat the seals and sealing surfaces with petroleum jelly.

-Turn the center housing with the piston bore facing up. Install the piston with a smooth motion while maintaining alignment. Misalignment and jerky motion, such as pounding, can cause seal damage. For best results, use three screw push blocks to press the piston into the bore maintaining alignment.

Installation sequence of correctly assembled seal.

Illus. 1 Free State of Installed Seal

Illus. 2 Correctly Sized or Heated Backup Ring - No Contact With Piston

Illus. 3 Correctly Installed Seal Assembly

Illustration. Installation sequence of correctly assembled seal.

Seals can be damaged if incorrect assembly methods are used. Seal damage will cause hard to detect leaks, and may also prevent proper piston retraction, leading to premature failure.

After the axle has been assembled, but not yet installed to the machine, test the brake piston seals for leakage. Install the necessary fittings on the brake pressure port to allow for an air pressure gauge rated at 0 to 2000 kPa (0 to 290 psi) and a shut off valve. Pressurize the brake assembly with approximately 690 kPa (100 psi) of air pressure. Hold this pressure at the brake piston with the shut off valve. The pressure drop at the gauge should not exceed 100 kPa (15 psi) in one minute. Repeat the test at lower pressures. Damaged seals may hold at one pressure, but leak at another. If the pressure drops at an unacceptable rate, check the gauge fittings for leaks with soapy water and correct as needed. If pressure continues to drop at an unacceptable rate you may have used too much petroleum jelly at assembly. Excursive the piston seals by applying and releasing the air pressure several times. If the pressure still drops at an unacceptable rate, there is a problem with the assembly and repair will be required.

For machines equiped with piston retraction springs and two piece seal assemblies, after determining the piston seals hold pressure, for machines using the seal assembly with backup ring, validate brake piston actuation and retraction. Remove the brake inspection port plug. Apply air pressure and the piston should actuate. Release air pressure and the piston should quickly retract 0.5 mm (.02 in). If the piston does not retract there is a problem with the assembly and repair will be required.

Machine Applications At Risk:

Certain applications such as mill yard logging, where there is repetitive braking from high speeds several times a minute, will require brake cooling. For most machines placed in these kind of applications, there are attachment brake oil coolers available.

NOTE: Do not consider attachment brake cooling as a solution to correct operator errors. Brake oil cooling will not overcome some conditions created by certain operator errors such as repetitive retarding to control machine travel speed down slopes.

Correcting a mechanical condition or an operator error may resolve the problem without the use of an attachment cooler. Before considering the use of an attachment brake oil cooler, use the information in this publication as a checklist.

-Are there any mechanical conditions needing repair?

-Are there any operating errors to correct?

-Are there any lubrication problems to correct?

-Are there any maintenance or servicing problems to correct?

After making any needed corrections follow the BRAKE OIL COOLER GUIDE below to determine if a cooler is required.

Brake Oil Cooler Guide

Towards the end of the shift after at least six hours of operation, slowly remove the axle housing oil fill plug and place a calibrated temperature probe directly into the oil. Hold the probe in the oil until the temperature reading stabilizes. Take this reading several times on different days each time after the same hours of machine operation. Take several measurements for each application the machine is placed into and several measurements for each different operator for the machine.

If the axle sump oil temperatures are fairly consistent, average them and compare with the chart below to determine if a cooler is required. If the numbers are far from being consistent, determine what application or operator produces the highest axle sump oil temperatures and see if that application or operating practice can be changed. If changes are made, run several temperature checks again before considering a cooler. If changes can't be made, average the highest temperatures recorded and compare with the chart below to determine if a cooler is required.

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