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Bull dozers and hydraulic excavators are track-type machines that form an integral part of any construction and mining site. Dozers have the toughest job in any mine, as they have to prepare haul roads often without any help from other machines, prepare the face for an excavator or a dump truck to load and haul respectively, maintain and clear the dumping area, move material and accumulate for a loader to load, and in areas where blasting is prohibited it has to perform ripping operation as well. The powertrain of a dozer is so designed that the engine torque gets multiplied at the torque converter, the transmission and the final drive before it reaches the sprocket and provides the brute pushing ability. In fact, the entire productive characteristics of a dozer is because of its tractive ability and hence the undercarriage components are the most affected one. While an excavator uses its travel capabilities only to shift itself from one location to the other, the load on its undercarriage system is not as dramatic unless the machine is used in specific operations like granite, marble and blue metal quarries.

An undercarriage with all its components accounts for approx 20 per cent of the machine cost, whereas it can represent almost 50 per cent of the machine maintenance cost. Hence, it is important to understand how undercarriage works and wears so that proactive methods can be implemented to enhance longevity and also predict wear. The track pads bite the ground and transmit the force it receives from the sprocket through the track links to push the dozer forward along with the blade load. All the components involved; the sprocket, links, pins and bushings, track pads and its grousers, track rollers, carrier rollers and idler are all subjected to wear as it moves on the ground and rubs with it or due to the relative motion between components (Figure 1).

Wear of undercarriage components cannot be eliminated, as they are always engaged with the ground and the abrasive character of soil can only hasten it. The wear and tear due to slippage in the relative contact areas between the sprocket and the bushing outer circumference, the pin and the bushing inner surface, the track roller and the link tread, the idler in contact with the track link will happen by default. The yardstick for normal and abnormal undercarriage wear of a mining dozer is very subjective and is purely based on the past experience and perception of the user. The rate of wear depends on the shoe selection, the nature of application, operation techniques and maintenance. Since several factors collectively determine the wear rate, manufacturers are unable to fix an assured life for undercarriage. However, a bit of caution and care can definitely retard wear and enhance the life of undercarriage components.

Correct operation and maintenance to enhance undercarriage life
Few dos and don?ts which operators and maintenance team must follow to enhance undercarriage life:
a)Do not allow the shoe to slip due to undue heavy load. The maximum drawbar pull exhibited by a dozer depends on the weight of the machine and the co-efficient of traction and not necessarily by the transmission or engine capacity. Hence, if the ground condition is very hard (co-efficient of traction will be poor) the shoe will slip at relatively low loads. When the shoe slips, there is zero productivity, only shoe wear and fuel consumption. Hence, if the shoe slips, the load should be reduced. Modern dozers like the Komatsu D375A-5 and above are provided with track shoe slip control to reduce track wear. During sharp turns or pivot turns, shoe slippage is high and hence slow long turns should be preferred wherever possible.
b)Do not impose load only on one track over a long period of time. When the dozer works on an incline, heavy load is imposed on one side of the track rollers and there is uneven wear. It may also happen that at certain sites there may be only left or right turns. This will also cause biased wear of rollers. Mine planners should design job activity such that there are turns on both sides. If nothing can be done, then the track rollers on either side should be swapped.
c)Do not drive the machine at high speed unless necessary. High speed travel will speed up wear as the sprocket and bushings, links and rollers, links and idler collide with each other heavily, causing damage. Contrary to common perception, the track parts wear faster when travelling in reverse, than forward.
d)Maintain proper track tension and keep it clean. Too tight a track will impose stress on links and bushings. Rocky dust and earth ingresses will speed up wear as it will act like emery paste between moving parts. Too loose a track will give rise to snaky track movement causing the pins go out of alignment and rollers to come out of track links. This will also aggravate the wear on carrier rollers and idlers while damaging the sprocket teeth. Keeping the tracks clean will also help in enhancing life of undercarriage components. When mud sticks and packs between the sprocket teeth, the sprocket pitch increases and a mismatch takes place.
e)Park the machine on level ground after the day?s work. If machine is parked on a steep slope after completion of work, thrust load will be imposed on the undercarriage in the direction of slope causing the O-rings of the seals in the idler, track rollers and carrier rollers to get deformed, initiating leakage.

Repair/Replacement
Even though correct shoe selection and operation will prolong the life of undercarriage components, timely and proper repair and rebuilding is a must to reduce the operating cost of a dozer. Explanation of some terms and a bit about the construction of the sprocket and the track link assembly would be relevant here.

a)Track pitch or link pitch (the distance between pin centres) and sprocket pitch (the centre distance between engaged teeth at the pitch line) are of equal distance on new components (Figure 2a).
b)Backlash is the clearance on either side of the bushing, between it and the sprocket (Figure 2b). This clearance permits a build-up of dirt before interference between the bushing and tooth of the sprocket takes place. However, it also effects a self-cleaning action to reduce the tendency of dirt to build-up in the root of the sprocket.
c)On the sprocket teeth, the wear pattern is complex and wear of different areas happen during different position of travel (Figure 3).

To understand wear characteristics of an undercarriage, it is important to understand the relationship between the link pitch and the sprocket pitch (Figure 4):
a) When new link pitch is equal to sprocket pitch
b) When wear occurs link pitch extends whereas sprocket pitch reduces
c) The above phenomenon causes a mismatch and can accelerate wear of bushing and sprocket
d) This has a cascading effect on all the components of undercarriage

It is interesting to note that, while the sprocket teeth wears on either side through this reversal or direction of operation, the internal pin to bushing wear is always on the same side. It might sound strange but it is true (Figure 5).

The link pitch elongation, sprocket pitch mismatch, internal pin and bushing wear has been the biggest factor in limiting undercarriage life. This means that when the pin and bushing is worn out completely, its other half remain untouched. Turning of pins and bushing by 180o can be a method to use their full service capacity. While the external wear of the bushing can be measured directly, the internal wear can be measured by the elongation of link pitch (Figure 6).

As the wear rate of sprocket, pins and bushes are linked, their repair or replacement should also be linked and thus timed together. If sprocket segments are to be replaced as it reaches its wear limit, then the pins and bushings should also be turned at the same time, or else the mismatch will remain and the new sprocket will not last as long as the previous one. Welding material on the worn out sprocket teeth to rebuild back to initial shape and dimension is followed at several jobsites. The profile of the sprocket teeth is quite complex and hence replacing the sprocket rim for a welded type sprocket or replacing the segments for a bolt on type is a better repair method and should be adopted. The other undercarriage components like the track roller, carrier roller, idler, and track pads will wear based on the ground condition and operation but will not affect the others as much as the sprocket and link - pins and bushings. Undercarriage components are expensive and their replacement consumes both time and labour considering the tools and facilities available at sites. Hence, measuring wear to understand wear rate, and to accurately calculate the balance life before the service or repair limit is reached is a specialised activity that has to be followed diligently. Major machine manufacturers have data for new components and their repair limits, and their field service teams are trained to perform the checks and advice customers appropriately. Ideally, the undercarriage repair schedule should be somewhat as given in Figure 7.

Komatsu has an elaborate undercarriage inspection programme (KUC) which is used by L&T service teams to measure and evaluate undercarriage health. Real time measurement is carried out at jobsites every 1,000 hours of operation and values fed into the system. Based on the values keyed, a wear percentage trend line can be created for each component and the balance useful life can also be captured. It helps schedule repair activity and plan parts inventory such that the required items reach jobsite just in time, when the machine needs it. Repair should be carried out when the wear is close to the repair limit, and should not be stretched to the service limit which may result in catastrophic failure and repair is not possible (Figure 8).

Running the undercarriage to destruction or to the end of service life isn?t also a bright idea as the drawbar pull, which is the dozer?s key performance parameter, drops drastically in the area beyond the repair limit. For example, as the grouser of track pad wears, the grouser becomes a little wider at the lower half and its biting ability decreases, adversely affecting the drawbar pull. The lowermost part of grouser is not hardened and once worn out to that level, there is drastic increase in the wear rate and the dozer ceases to be productive. Hence, timely lug bar welding will not only keep the dozer doing its duty, but life of track shoe is also enhanced.

Getting the best result
The knowledge of how undercarriage works and wears, careful operation, regular maintenance, periodical checks and timely repairs using genuine parts based on wear trend analysis is the way to get the best out of undercarriage components.

1. Forward drive side wear- forward operation
2. Reverse drive side wear- when operating in reverse
3. Forward drive side tooth tip wear - result of increased track pitch
4. Root wear - result of the bushing sliding from side to side on the sprocket
5. Rotative wear - result of bushing rotation as the bushing leaves or enters the sprocket
6. Reverse drive side tip wear - result of sprocket pitch which is greater than the track pitch and vice-versa

Article courtesy: Larsen & Toubro Construction Equipment