Maintenance Guide For Cab Pressurization on All Caterpillar Products{0374, 0644, 0768, 7301, 7311, 7500} Caterpillar

Caterpillar Products:with Cabs All

Introduction

Maintaining positive cab air pressure over the life of the machine helps ensure operator comfort, operator productivity, and increased cab cleanliness. Positive cab air pressure will reduce operator exposure to airborne particulates. Cab air quality can decrease over time from normal wear or physical damage.

This document provides background information, measurement techniques, guidelines for inspection, and corrective action to maintain appropriate cab air pressure. These practices apply to a broad range of cabs and machines. The illustrations provided are to be used for reference regardless of machine model.

Product Enhancement Description

Useful Definitions

Cab pressure is the difference in air pressure between the inside of the cab and the external environment.

Cab airflow is the total airflow experienced by the operator that is ejected from the air vent louvers.

Cab Fresh or ventilation airflow is the fraction of the total cab airflow that is brought in from the outside. This airflow is what creates the cab pressure.

Cab return or recirculation airflow is the fraction of the total cab airflow that is pulled from the environment of the operator. The airflow is again cooled or again heated, and then again rejected from the air vent louvers.

Cab pressure control principles

Illustration 1	g03172796
Positive cab pressure prevents dirt entry

The purpose of the cab pressurization system is to create a clean operating environment. This pressurization system is accomplished by preventing unwanted outside air from entering the cab through openings by creating a positive pressure inside the cab. Furthermore, the desired outside air that is drawn in to create that pressure is filtered.

The amount of cab air pressure required to prevent dirt entry through openings in the cab is small. Therefore, the air pressure is typically measured in Pascal (Pa) units.

Illustration 2	g03172822

Total cab airflow is related to cab pressure since a portion of total airflow is drawn into the cab to create positive pressure, and maintain a comfortable temperature inside the cab. The total cab airflow is made up of fresh air (B) drawn into the cab heating or cooling unit from outside and recirculation air (C) drawn into heating or cooling unit from the cab environment. Refer to Illustration 2. The fresh air is drawn in by the heating and cooling system through the blower. At startup, fresh air is drawn in at an increasing rate until the cab leaks an amount of air (A) which is equal to the air being drawn in. The process occurs in a matter of seconds.

Cab pressure inspection checklist

The performance of cab pressure-related components can degrade with aging, wear, exposure to extreme conditions, physical damage, and so on. Regular inspection of the cab and machine can be used to identify degradation of cab pressure-related components contributing to poor cab pressurization. The affected parts can be repaired or replaced as needed.

Inspection Checklist

1. Measure cab Pressure
   • Proceed to Step 2, if cab pressure on high blower speed is below 0.05 kPa (0.01 psi).

   • Proceed to Step 3, if cab pressure on high blower speed is above 0.20 kPa (0.03 psi).

2. Low cab Pressure Inspection
   • Fresh air filter is free of debris and is not saturated with dust.

   • Recirculation filter is in place.

   • Filter seals are intact and compressed evenly by doors / covers / latches.

   • Door seals are compliant and have no gaps, misalignments, or leaks.

   • Door latch adjustment and wear provide tight, rattle-free, leak free sealing.

   • Fixed window seals are in-place with no gaps, including bonding.

   • Sliding windows are properly aligned with no channel obstructions.

   • Latches secure windows.

   • Hinged window seals are complete, compliant, and seal with no leaks.

   • Hinged window latch adjustment and wear enable tight, rattle-free closure.

   • Exterior access panels are in-place, tightly secured, and properly sealed.

   • Exterior fixed panels are sealed / caulked with no damage ( holes, cracks, missing pieces).

   • Cooling and heating coils are free of debris and ice.

   • Cooling and heating coil fins are straight.

   • Blower blades are undamaged.

3. High cab pressure inspection
   • Fresh air filter is in place.

   • Recirculation air filter is free of debris and is not saturated with dust.

   • Filter seals are intact and compressed evenly by doors / covers / latches.

   • Air path seals are intact and compressed.

Measure cab pressure

Illustration 3	g03172876
Measuring cab airflow with digital manometer

The cab pressure differential manometer can be used either inside or outside the cab. If the manometer is read from outside the cab, run the high-pressure port to the inside of cab. If the manometer is read from inside the cab, run the low-pressure port to the outside of cab. Illustration 3 shows the manometer outside the cab with the high-pressure line running into the cab.

The high-pressure line can be run through the door seals, grommet, and so on. Take care to make sure that the line does not become pinched. Place a small and rigid tube in the end of the line.

Also take care of manometer port placed inside the cab, so that the port is not affect by the high airflow velocity from the cab air louvers.

The digital manometer selected for the test should have a resolution from 0 to 0.25 kPa (0 to 0.04 psi) and calibrated yearly.

A lower-cost alternative for the measuring device would be to use a liquid filled manometer.

Contact the Caterpillar Dealer Solution Network (DSN) for specific tool recommendations, as there are currently no dealer service tool offerings.

Cab Pressure Component Inspection

The section describes inspections and corrective actions. The cabs may differ in designs and cab sealing features. Some features listed below may not exist on some cabs.

HVAC

Illustration 4	g03172918
HVAC system (1) Inspect the HVAC components

The HVAC system contains many components that are integral to the cab pressurization system. Including fresh air and recirculation filters, heater and evaporator coils, blowers, and seals.

The fresh air filter is the most common culprit for low cab air pressure. The fresh air filter can become clogged with heavy use or if large debris becomes trapped in the air intake. Typically, the fresh air filter should be replaced every week (40-50) hours. The recirculation filter should be replaced monthly (160-200) hours. However, the need to change the filters can vary based on the dust concentration in the environment.

The heating and cooling coils are typically made of copper tubes and aluminum fins. The fins are thin to promote good heat transfer, but fins can be bent over and damaged easily. The fins can be repaired with a fin comb (208-1350). Also, water condensation can freeze and block the airflow if the thermostat in the evaporator is either misplaced or not working.

Illustration 5	g03172927
Conditioner unit (2) Filter (3) Blower (4) Coils (5) Seals

The blower may become damaged, if debris becomes clogged in the blades. Ensure that the blower functions properly.

Lastly, the conditioner unit contains seals (5) to connect to the cab structure and / or ductwork. The seals (5) may become worn or misaligned. Repair or replace the seals as necessary.

Doors

Illustration 6	g03172930
Broken perimeter seal (6)

Illustration 7	g03172931
Maintain the door seal around the entire perimeter Proper perimeter seal (7)

A door is the largest potential air leak path in a cab. Perimeter seals on doors are used for weather stripping and noise control besides air sealing. On many cabs, the seal extends around the entire perimeter. Inspect the seal for excessive wear, misalignment, or damage. Repair or replace as necessary.

Illustration 8	g03172932
Door latch (8)

Illustration 9	g03172933
Adjust the striker to obtain proper door seal compression Proper compression (9)

The latch system is critical to ensure proper compression when the door is closed. Inspect the door in the latched position from both outside and inside the cab. The door should compress against the seal uniformly around perimeter of the door. If compression is too loose, adjust or replace the latch and / or striker to improve compression.

Some doors have a two-stage latch. With moderate closing force the latch engages first stage of latch. Stronger force is required for the door to engage the second stage. The sealing performance of the door is based on compression at the second stage.

Windows

Illustration 10	g03172934
Loose zip-in seal (10) Cracked glass (11)

Illustration 11	g03172935
Repair or replace broken windows and damaged seals

A window is part of the operator enclosure and is required for proper cab sealing. Some windows are fixed in place, such as the windshield. Other windows are designed to move for ventilation or to communicate to others on the ground. Windows should be free of cracks and holes and should seal well around the perimeter. Repair or replace any windows with holes or cracks.

Ensure that zip-in seals are in good condition and properly set. Repair or replace as necessary.

If replacing a bonded window, ensure that the bond covers the entire perimeter of the window. The bond should be allowed to cure properly before placing into service. Improper cure could result in changes to the window sealing properties.

For a sliding window, the channel should be clear of dirt and debris that may cause window misalignment. Ensure that the latch can hold the window securely in place. Clean the channel and adjust or repair the latch if necessary.

Illustration 12	g03172956
Open latch (12)

Illustration 13	g03172957
Make sure that windows are fully latched to get best pressurization Properly latched (13)

For hinged windows, inspect the condition of the perimeter seal. When shut, there should be no gaps between the window, the seal, and the window frame. If displaced or worn, repair or replace as necessary. Ensure that the latch can hold the window securely in place and can be easily operated. Adjust or repair the latch if necessary.

Penetration

Illustration 14	g03172958
Cab penetration (14)

Illustration 15	g03172959
Plug holes with grommets or caulk Grommet or Caulk (15)

Illustration 16	g03172960
Hole in boot (16) Cracks in boot (17)

Illustration 17	g03172961
Inspect rubber boots

Some cabs have penetrations for wiring harnesses, hoses, and so on along with various means to seal the holes ( caulk, grommet, and so on). The penetration holes may be located on the floor or side panels. Inspect the areas and repair or replace the seals as necessary.

Exterior Panels

Illustration 18	g03172976
Panel removed (18)

Illustration 19	g03172977
Properly reinstall cab components

Access panels may be found on the sides and floor of the cab. The panels should be securely in place to provide an airflow path for the conditioned air in some machines.

Inspect cab panels for holes or cracks. Repair or replace as necessary.

Some panels may have weather stripping, caulk, or a seal around part of the entire perimeter. If so, check that the seal is in good condition and has not been displaced or worn. Repair or replace as necessary.

Interior Panels

Illustration 20	g03172978
Cracked panel (19) Damaged hole (20)

Illustration 21	g03172979
Properly repair cab panels Caulked crack

Poor sealing access panels inside the cab may also lead to low cab pressure. There may be hidden service access panels under the floor mat, behind the seat, and so on. The poor sealing can be sealed with bulk material with adhesive backing. Skip welds inside the cab structure can be caulked to improve sealing.