Undercarriage

Track chain

Track chain is made up of many track sections and each track section is made up of two track links, a hardened pin, and a bushing. These individual track sections are interconnected to the link assembly (see Figure 1). The two track links used in each section have provisions for attaching a track shoe and also provide a rail for the track rollers to maintain accurate track alignment. Sealed tracks have a solid pin. Sealed and lubricated tracks have a hollow pin, which provides a path for lubricating the pin and bushing of the next track section. When installing a center-drilled pin, the cross drill hole must be installed toward the rail of the link, which keeps the pin in compression to resist the possibility of crushing. The pins and bushings are press-fit into the links.

Fig 1. Track chain

Fig 2. Track shoe

Idlers

Depending on the track design, the equipment could have one or two idlers per track section. If the equipment has an elevated sprocket system, there will be two idlers: a front idler and a back idler. Equipment that has a conventional oval track requires only one idler. The purpose of an idler is to help guide the track through the track rollers and to help support part of the weight of the equipment. Figure 3 shows an example of a typical idler assembly. Besides providing alignment, the idler maintains the correct tension and slack on the track. The tension on the track is adjusted by moving the idler back and forth on the track frame with the use of hydraulic pressure.

Fig 3. Idler

Equipment with oval tracks can have a two-position idler, with one position that is used when drawbar work is required. This position minimizes the amount of track that is in contact with the ground, resulting in less track wear. When equipment must operate with heavy implements attached to the front, the use of the lower mount for the idler places more track on the roadway; this makes the equipment more stable but accelerates track wear. Too much track tension con-tributes to accelerated wear due to increased loading on the track components.

Drive sprocket

The sprocket, as shown in Figure 4, is not mounted to the track frame but is attached to the final drive. Its job is to transfer drive torque to the track. The teeth on the outside of the sprocket act like gear teeth. They engage the track links and propel the equipment on the continuous track loop. Older equipment often has a one-piece sprocket assembly; the use of individual bolts on segments is a recent innovation. On older models, the old sprocket had to be cut off and a new one welded in its place. Many equipment owners replaced the welded version with a weld on adapter ring; a new sprocket could then be bolted on, saving a considerable amount of time. Operating on different types of terrain requires that the segments be quickly interchangeable.

Fig 4. Drive Sprocket

On modern equipment, changing sprocket segments is quick and easy; position the track so that the segment that requires changing is on the inside (away from the track links), unbolt the old segment, and bolt in and torque the new one. Rotate the track a few feet and repeat the procedure until all the segments are replaced.

Rollers

Modern track design utilizes two different types of track rollers to maintain track alignment. The bottom rollers are track rollers; they support the weight of the equipment and ensure that the weight of the equipment is distributed evenly over the bottom of the track. The track rollers are spaced closely together, and generally a large number of them are mounted to the bottom side of the track frame.

The two types of track roller designs are single flange and double flange. Single flange rollers are located closest to the sprockets. Double flange rollers maximize the track stability and alignment. The surface of the track rollers is hardened to the same Rockwell scale as that of the track links. Cooling and lubrication are provided by the oil sealed in the housing. Because of the twin flanges, double flange track rollers are used to maximize track alignment. There are several design configurations of track rollers. Figure 5 shows the parts breakdown of a typical Caterpillar track roller.

Fig 5. Parts breakdown of a typical Caterpillar track roller

Carrier rollers are located above the frame rail. The main purpose of the carrier roller is to support the weight of the top portion of track as it rolls between the idler and the sprocket. These rollers normally have a single flange at the center of the roller that helps in controlling track sag and whipping during operation. A secondary function of the carrier rollers is to maintain proper alignment on the upper section of the track between the idler and sprocket.

These rollers also have a hardened surface that has equal hardness as that of the track link surfaces; this can have a slightly negative impact on the life of the track links. Some equipment manufacturer cantilever mount the carrier rollers to reduce material build-up.

Figure 6 shows an exploded view of a typical carrier roller. The retaining ring on a carrier roller fastens the end collar to the roller shell and is held in position on the support shaft by a groove. The bearings are held in place in the carrier shell by the end collar and seals, which also prevent oil from leaking out of and the dirt getting into the bearings. The two roller bearing assemblies allow the carrier roller to freely roll on the shaft. The shaft has mounting points to secure the carrier roller to the track frame. A plug and O-ring seal the lubrication opening in the shaft. The retainer plate, which mounts to the shaft, holds the tapered roller bearings inside the carrier housing. A roller cover provides a seal on one end and is fastened with bolts to the end of the carrier shell.

Fig 6. Exploded view of a typical carrier roller

Tires and Rims

This section describes the most common types of tires and rims found on off-road equipment. Maintenance and replacement procedures can vary greatly between different types of tire and rim combinations. The explanations and examples provided in this section cover general practices and do not replace the specific manufacturers’ and site-specific procedures that must be followed when servicing tires and rims. Off-road tire and rim requirements on equipment vary greatly and are dependent on factors such as ground surface conditions. Steep hills, along with loads and speed, influence the type of tires chosen.

The cost of off-road tire replacement can add a significant expense to the work performed if the appropriate tires are not selected and properly maintained. Selecting the appropriate tires and rims for equipment and work performed will result in longer tire life, minimizing down time as well as reducing tire costs. The ply rating and tread type also play a role in safe, efficient operation of the equipment. Maintaining correct operating procedures observing speed and load limitations results in maximum effectiveness with minimal tire damage as well as preventing unscheduled down time

Heavy Equipment Undercarriage General Maintenance

• Inspect undercarriage and components
• Demonstrate suitable use of ground engaging equipment
• Perform track tension adjustments
• Demonstrate appropriate blocking/cribbing techniques.

References

1. Technical book, “Construction Machinery Training”, Instate, Imlambad
2. Harris, F. and McCaffer, “Management of Construction Equipment”, Macmillan Education Ltd. London, UK.
3. Erich J. Schulz, “Diesel Equipment I and II”, Mcgraw-Hill book co.
4. Frank Harries, Ronald McCaffer, “Construction of Plant Excavating and Material Handling”, Granda Publishing.
5. SAE Handbook Volume 4
6. “Caterpillar performance Handbook”, Edition 33, Caterpillar Inc, Peoria, Illinois, USA.