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What is the purpose of a lubrication pump in a tandem drive axle set? There is a theory that this pump sends lube to the outer ends, this is not true. It has been reported that this pump is required on steep inclines, also not true. Yet another opinion is that over the road chassis do not need the pump. Based upon the information below, we will let you decide about the last one.
The rear axle pump option is located in the forward drive axle. Without the pump the ring gear splashes lubrication up to the gears. As long as the ring gear is spinning forward, it does a fantastic job of accomplishing this. Which is why the pump is not standard. However, without a pump there is no way to ensure lube to the upper gears and bearings in the power divider when the rearmost tandem axle is spinning, and the forward outer ends are not. This condition spins the interaxle components, but not the ring gear which supplies splash lube throughout the housing.
If a driver stops and locks the axles together using the interaxle switch, when the chassis is in inclement weather, then the pump is not required because at least one outer end on forward drive axle will be spinning and the splash from the ring gear action will be sufficient. However, relying on a driver to take this action could be risky.
The pump provides a nice insurance policy for drivers who do not understand when they should lock the axles. Another condition that might warrant the pump is in applications where the truck backs long distances. For example, a dump truck hauling asphalt. Ordering 6S/6M also helps resolve this condition and should always be ordered with ATC.
Not all axles have a pump option. For newer axle designs the Engineers have devised a way to keep the gearing lubricated in all events. When the option exists, the operating principle differs. Some manufacturers operate this pump all the time, others only when there is a difference in forward vs rear axle speed. Some axle manufactures also add an external filter, which requires maintenance.