- Motor Grader:
- 120G (S/N: 4HD1-UP; 1LK1-UP; 87V1-UP; 11W1-UP; 61W1-UP)
- 120H (S/N: 4MK1-UP; 5FM1-UP; 6NM1-UP; 6TM1-UP; 2AN1-UP; 9FN1-UP; 3GR1-UP; 9YR1-UP)
- 120M (S/N: B9C1-UP; B9N1-UP; B9W1-UP; D9W1-UP)
- 12G (S/N: 3WC1-UP; 3PL1-UP; 61M1-UP)
- 12H (S/N: 4XM1-UP; 5ZM1-UP; 8MN1-UP; 2LR1-UP; 2WR1-UP; 4ER1-UP; 2GS1-UP)
- 12M Series 3 (S/N: N9B1-UP; N9F1-UP)
- 12M (S/N: B9F1-UP; B9R1-UP)
- 130G (S/N: 74V1-UP)
- 135H (S/N: 3YK1-UP; 7MM1-UP; 8WN1-UP)
- 140G (S/N: 5MD1-UP; 50H1-UP; 72V1-UP; 13W1-UP)
- 140H (S/N: 2ZK1-UP; 5HM1-UP; 8JM1-UP; 8KM1-UP; 9TN1-UP; 9ZN1-UP; 3FR1-UP; 3AS1-UP)
- 140M Series 2 (S/N: M9D1-UP; R9G1-UP; M9J1-UP; R9M1-UP)
- 140M Series 3 (S/N: N9D1-UP; N9J1-UP)
- 140M (S/N: B9D1-UP; B9G1-UP; D9G1-UP; B9M1-UP)
- 143H (S/N: 1AL1-UP)
- 14G (S/N: 03L1-UP; 96U1-UP)
- 14H (S/N: 7WJ1-UP)
- 14L (S/N: B941-UP)
- 14M (S/N: B9J1-UP; R9J1-UP)
- 160G (S/N: 4JD1-UP)
- 160H (S/N: 9EJ1-UP; 2FM1-UP; 3GM1-UP; 6WM1-UP; 9JM1-UP; 3HR1-UP; 2HS1-UP)
- 160M Series 2 (S/N: M9E1-UP; M9K1-UP; R9L1-UP; R9T1-UP)
- 160M Series 3 (S/N: N9E1-UP; N9K1-UP)
- 160M (S/N: B9E1-UP; B9L1-UP; B9T1-UP; D9T1-UP)
- 163H (S/N: 5AK1-UP)
- 16G (S/N: 93U1-UP)
- 16H (S/N: 6ZJ1-UP)
- 16M (S/N: B9H1-UP; R9H1-UP)
- 24M (S/N: B9K1-UP; B931-UP)
- 120H (S/N: 4MK1-UP; 5FM1-UP; 6NM1-UP; 6TM1-UP; 2AN1-UP; 9FN1-UP; 3GR1-UP; 9YR1-UP)
Introduction
For several years dealers have experienced motor grader "bounce". Motor Grader "bounce" is not uncommon nor exclusive to Caterpillar Motor Graders. However, certain machine characteristics can cause the phenomena, as well as job operating conditions.
The purpose of this special instruction is to provide a reference for dealer personnel who may become involved in a motor grader bounce problem. Some background, references, on-site testing experiences, and other material are provided to assist in the resolution of this rather elusive problem.
Types of Machine Motion
The following definitions are provided to familiarize the reader with terms used in the study of motor grader bounce.
- Bounce is when the entire motor grader moves in a vertical direction while traveling down a roadway.
- Pitch is caused when one end of the machine moves further in a vertical direction than the other end of the machine. Pitch occurs with motor grader blade scallops. The rear of the machine moves up and down more than the front due to the rear tires moving over the blade scallops.
- Roll is when the motor grader moves from side to side. As with pitch, roll occurs with motor grader blade scallops. The difference, however, is that the right and left tires do not travel over the scallops at the same time. This type of motion has been referred to as "Duck-Walk". In the case of blade scallop, it is possible for roll to dominate the overall machine motion.
Definitions and Explanations of the Types of Motor Grader Bounce
There are two distinct types of motor grader bounce.
- Low-Speed: Blade Scallop or In-The-Dirt Bounce.
- High Speed: Roading Bounce or Motor Grader Lope.
In the past, much confusion and miscommunication resulted from not keeping the two bounce phenomena separate. As will become apparent in the following detailed descriptions of these types of bounce. The mechanism representing the two phenomena is similar, but the potential solutions to each have little, if anything, to do with the other.
Low-Speed: Blade Scallop or In-The-Dirt Bounce
An example is a road maintenance application where the machine is making the final pass spreading a windrow across the crown of the road. The front tires travel over a bump or pothole in the road that causes the blade to rise or fall. The rise or fall forms an initial wave or scallop in the material being spread. The tandem tires travel over the scallop causing the blade to form yet another scallop. Thus the rear tires feed back a signal to the blade.
If the conditions described are present, the motor grader will continue forming scallops until the operator takes action. Possible actions include changing the ground speed or the attitude of the blade.
To understand the phenomena, it is necessary to view the motor grader the way an engineer studying this phenomenon views the machine. What the engineer sees is a large mass sitting on six rather weak springs (the tires). See Illustration 1 for machine visual.
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| Illustration 1 | g02714316 |
This mass is free to move, compressing, or extending these springs (tires). A first-hand feel for this spring and mass can be experienced by standing on the front axle or the ripper of a parked machine with the blade off the ground. By bending and straightening your knees, you can start the machine moving vertically. Do not try to force the motion but encourage the machine to move as the machine wants to, you can experience the bounce.
This spring/mass demonstration illustrates two key points with respect to bounce. First, how little effort is required to start and maintain the machine motion. Second, for the machine motion (bounce) to be significant, the motion must be sustained. That is, the motion must continue until the machine operator does something to stop the motion.
Following are three factors which must be present to allow a sustainable blade scallop.
Blading conditions
- The motor grader must be spreading material that flows well (for example, dry sand).
- The blade load must be relatively light but adequate to spread material under both sets of tandem tires.
- There must be a hard surface underneath the spread material.
Obviously, since little can be done about the material being bladed, this first factor is, usually, not within control. The operator must make the best of what there is to work with.
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| Illustration 2 | g02714390 |
Position of the blade with respect to the tandem tires
See Illustration 2, if the distance between the blade and the front tandem tire (A) is the same as the distance between the tandem tires (B). Then the front and rear tandem tires will roll over the scallop peaks at the same time. The full effect of the scallops will be fed back to the blade so that another scallop will be formed.
Coversely, if the distance between the blade and front tandem tire is such that the front tandem tire rolls over a scallop peak when the rear tandem tire is at a scallop valley. Then the tandem effectively filters the blade scallops. The blade will not move up and down, and consequently, no more scallops will be made. Thus the position of the blade with respect to the tandem tires is a key factor in determining if a sustainable bounce is possible.
Machine speed
For the bounce to be sustained, the up and down movement must be at the bounce resonant frequency - about 2.5 times per second. This means that the tandem tires must roll over a scallop peak about 2.5 times per second. Thus, for a given distance between scallops, a certain machine speed is required to obtain a sustainable bounce.
The distance between scallops is determined by the location of the blade with respect to the tandem tires. The operator often has little leeway in setting the blade position since the operator needs to grade the road with the fewest number of passes. Consequently, of the three factors necessary for sustainable scallop, the one most easily changed is the speed of the machine.
Blade Scallop Implications For Particular Motor Graders
All motor graders will form blade scallops under certain conditions. The three necessary factors for sustainable blade scallop presented above are equally applicable to all motor graders. Machine characteristics that determine the specific conditions that make a particular motor grader susceptible to blade scallop bounce are: weight distribution, axle spacing, blade base, tandem tire spacing, type of tires, and tire inflation pressure. The following are some bounce characteristics of specific machines.
Contributing factors to bounce are:
- Machine Configuration
- Ground Speed
- Engine RPM
- Gear of Operation
Alternating any of the above factors can reduce Motor Grader scallop or bounce.
For machines with this configuration, the simplest solution is to back off the throttle slightly. Reducing the engine RPM to somewhere between three-quarter and seven-eighths throttle is enough to eliminate the machine speed factor necessary for blade scallop.
Many experienced operators use the inching pedal as a way to respond to scallop initiation. As soon as the operator detects scalloping, they step on the decelerator long enough to break up the oscillation. Then let off the decelerator to resume the previous speed. The disadvantage to this procedure is that one or two scallops have been formed prior to stepping on the decelerator. Operating at slightly less than full throttle usually avoids scalloping altogether.
If reducing speed is judged to be unacceptable, then changing the blade position is the next simplest thing to do. Rotating the blade, changing the blade tip, or changing the centershift position can be effective in eliminating the blade position factor necessary for blade scallop.
If neither of these two suggestions are acceptable, then adjusting tire inflation pressures will reduce the size of the scallops. AWD machines will not apply to this tire inflation procedure. Traction and tire life can be affected by changing tire inflation pressure. This should be kept in mind when evaluating changing tire inflation pressure. Specific applications and machine configurations may disallow changing the inflation pressure. The tire supplier should always be consulted before deviating from the recommended pressure.
Installation of an implement can affect weight distribution of the motor grader which is another way to reduce the size of the scallops. Installing a ripper/mid mount scarifier versus a front scarifier will reduce blade scallop.
An effective combination to avoid sustained blade scallop when the blading condition factor favors bounce is: Change the centershift cylinder length by approximately 254 mm (10 inch) to shift the drawbar in the direction the material is being cast, reference Illustration 3.
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| Illustration 3 | g02714441 |
High Speed: Roading Bounce or Motor Grader Lope
High speed bounce will occur more often when the motor grader is being roaded. The drawbar is raised and only the drawbar mass, which is a major contributor, is part of the phenomena. Either a bump or a pothole will start the machine moving vertically. If the tires and rims are out of round, the motor grader may continue to move vertically (lope or bounce) until some change is made. The change may be an adjustment or difference in machine speed or another bump (or pothole) that disturbs the oscillation.
To understand this phenomenon, the machine must be viewed as a spring/mass system as with the blade scallop bounce. What was said about the spring/mass system in that discussion is equally applicable here.
It is obvious that a machine moving down a road will experience an uneven surface. What matters is whether the spring/mass system will oscillate at the bounce mode frequency of the machine. If it does, it will take little to maintain the undesirable movement (bounce).
It has been found that tires and rims that are not perfectly round, but still within the engineering specifications, can provide the small amount of energy needed to sustain the vertical movement (bounce) when the motor grader is traveling at specific speeds. Recall the little effort required to start and maintain the spring/mass demonstration motion. If the tire and rim can be made round, or at least nearly round, then the energy will not be available to the spring/mass system and the vertical motion cannot be sustained.
For the purposes of motor grader bounce, the tire and the rim must be thought of as a unit. The contour of the tire and rim unit is dependent not only on the contour of the tire, but also the contour of the rim. Since an out-of-round rim can influence the amount the tire/rim unit is out-of-round, the out-of-round rim can be used to offset the out-of-round tire. The mounting process referred to as "match mounting" is used to make the contour of the tire/rim unit as round as possible.
The "match mounting" process consists of determining the high points of the tire and rim. The high spot of the tire is positioned over the low spot of the rim. The result is a rounder contour of the tire/rim unit.
Problem Description
Dealers have experienced Motor Grader bounce as result of the following:
- Specific tire models
- Out of round tire or rim.
- Operators understanding of bounce.
- Tire air pressure settings.
- Ambient temperature.
- Soil conditions
- Blade load.
- Seat suspension
Problem Resolution
Operator Understanding of Bounce
Another obvious input to bounce, could be the operator. Operators can reduce or eliminate most bounce problems by using a different technique. This action is the most important point: a good operator controls bounce by controlling the motor grader. Do not let the material control the machine, set up the machine to control the material.
Tire Air Pressure Settings
Tire inflation pressure has a major effect on bounce. Increasing inflation pressure increases the tire stiffness (spring rate). In general, increasing the stiffness will decrease the chance of bounce in the desired operating range of machine speed. Through experience with Motor Graders, inflating all the tires to 345 kPa (50 psi) will eliminate blade scallop (low speed or in the dirt bounce). Only in the case of specific machine configurations decreasing inflation pressure decrease the likelihood of bounce. Traction and tire life can be affected by changing the inflation pressure from the recommended pressure. The tire supplier should always be consulted before deviating from the recommended pressure. For this reason, the operator should temporarily increase or decrease the inflation pressure, then change the pressure back to the recommended value. It is recommended that the tires are inflated and deflated in a crossing pattern, reference Illustration 4.
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| Illustration 4 | g02714884 |
Alternate pressure across the tandems as shown. | |
Ambient Air Temperature
Motor grader tires are susceptible to "flat spots" in all ambient temperatures. A machine needs only 15 to 20 minutes to develop tire flat spots significant enough to cause bounce. The motor grader operator needs to be aware of this occurrence to ensure that a bounce complaint is not due to temporary tire flat spots. The operator also needs to be aware that the colder the tires are, the longer it will take to roll out any flat spots.
Seat Suspension
Another factor that was noted during resolution of bounce on several machines was the interaction of the operator, the seat suspension, and the throttle pedal. When the seat suspension was adjusted softly, the operator would tend to move vertically with any slight input from an uneven road surface. The adjustment in turn could also cause the operator to slightly lift and apply their foot to the throttle pedal which could also excite a bounce condition. Use of the hand throttle may be useful in some conditions. This was not highly discernible by the operator because he was in the seat and suspended, but it was noticed by the rider in the cab or by someone observing from outside. Set the seat suspension appropriately for the size operator in the seat. Stiffer seat suspension settings would be more desirable to minimize the operators perception of bounce. In order to ensure that the seat is suited best for the operator refer to Special Instruction, REHS2761, "Installation Instruction for 212-7779 Shock Absorber Kit, 192-7848 Shock Absorber Kit, and 213-5170 Indicator Kit on Toggle Link Vertical (TLV) Seat Suspensions".
Tire Size
Changing to a wide-base or bigger footprint tire can reduce motor grader bounce. A 17.50 × 25 tire is more likely to envelop a road bump or bridge a pothole than a 14.00 × 24 tire. Therefore, the road surface irregularity is less likely to be felt by the machine. Road surface irregularities instigate motor grader bounce.
Addressing Out-Of-Round Tires and Rims
The following process will address most out-of-round tire and rim units. Required equipment to measure total indicated runout is either a dial indicator or digital indicator, an indicator stand with a magnetic base, and paint pen.
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| Illustration 5 | g03833940 |
Digital indicator | |
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| Illustration 6 | g03833941 |
Dial indicator | |
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| Illustration 7 | g03833942 |
Magnetic base | |
- Verify that all tires are inflated to the recommended pressure. Inflation pressures that are too low can lead to incorrect runout values.
- Warm the tires by roading the machine for 5 to 10 minutes. Roading the machine will also eliminate any flat spots on the tires caused by the machine being parked.
- A machine that has been parked needs only 15 to 20 minutes to develop significant flat spots.
- To keep any new flat spots from forming on the tires, raise the tires off the ground as soon as possible after roading the machine.
- The tires can be raised off the ground using several jacks.
- Once the machine is up, be sure to use jack stands for safety and to prevent the tandems from pivoting
- For overall simplicity start with the two front tires.
- To measure the Total Indicated Runout (TIR) of each tire, you will measure the lug at the centerline of the tire.
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| Illustration 8 | g03833826 |
- Set the magnetic base on a block and mount the dial indictor on the magnetic base positioning it on the center line of the lug.
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| Illustration 9 | g03833825 |
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| Illustration 10 | g03833844 |
Close up view of Illustration 9 | |
- Mark this location as the starting point of the measurement process and "zero" the dial indicator.
- Retract the indicator and rotate the tire to the next lug
- Release the indicator to take the measurement.
- Write the measurement of the lug for simplified tracking on the exterior wall of the tire next to the lug.
- Repeat Steps 8 and 9 until the tire has been fully rotated.
- If a lug measures greater than 5 mm (0.2 inch), this is considered to be a high spot.
- Once that tire is completed, move to the next one and continue the process until all six tires have been measured.
- Take the two tires with the greatest TIR and place them on the front axle. The tires with the lowest TIR should be placed on the front tandem under the cab and the remaining two on the rear tandem.
- All jack stands should be removed and the machine lowered back to the ground
- Take the machine for a test drive to determine if the Roading Bounce is resolved.
- If the issue is resolved, return the machine to service.
- If Roading Bounce persists return the machine to the service area and repeat Step 3.
- Once the tires are off the ground, the TIR of the rim needs to be measured.
- To measure the rim, the tire has to be removed from the rim.
- Be sure to follow proper disassembly guidelines.
- Using a dial indicator, measure the inner diameter of the rim.
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| Illustration 11 | g03833395 |
- Find the rim weld seam (A) and mark 50.8 mm (2 inch) (B) on each side as you are not to measure that area.
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| Illustration 12 | g03833429 |
View of rim (A) Weld seam (B) 50.8 mm (2 inch) | |
- Mark a starting point out and "zero" the dial indicator.
- Retract the indicator and rotate the rim approximately 30°.
- Release the indicator to take the measurement.
- Write the measurement on the exterior of the rim.
- Continue rotating the rim and taking measurements every 30° until the complete rim is measured.
- If any point measures greater than 3.18 mm (0.125 inch), this is considered a high spot.
- If the machine you are working on is a 16M or larger, use 4.5 mm (0.18 inch) as the target value for the high spot.
- While the tire is removed, check the inner rim bead since this can cause instability.
- Position the dial indicator on the inner rim bead.
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| Illustration 13 | g03833476 |
- Mark a starting point out and "zero" the dial indicator.
- Retract the indicator and rotate the rim approximately 30°.
- Release the indicator and take the next measurement.
- Continue rotating the rim and taking measurements every 30° until the complete rim is measured.
- If any point measures greater than 3.18 mm (0.125 inch), this is considered a high spot.
- If the machine you are working on is a 16M or larger, use 4.5 mm (0.18 inch) as the target value for the high spot.
- If the inner rim bead exceeds specification, then the rim should be replaced, as this issue cannot be adjusted out.
- If the inner rim bead is within specification, then proceed to the next step.
- Once the rim has been measured at both locations, the rim measuring is complete.
- Referring to the numbers for the lug measurement and the inner diameter of the rim, rotate the tire so that the highest lug position is on the lowest rim position and remount the tire.
- Be sure to follow proper tire assembly guidelines.
- Repeat the rim measuring and tire rotation process until all rims have been checked.
- Once this process is complete, take the machine for a test drive to determine if the Roading Bounce is resolved.
- If the issue is resolved, return the machine to service.
- If Roading Bounce persists, but all rims were within specifications contact the tire supplier as tires may be out of round.
- If Roading Bounce persists, and all rims were not within specifications order two new rims to replace the two rims that were the farthest out of specification at the inner diameter measurement.
- Be sure to install the new rims on the front tandem under the cab.