Copyright Goodheart-Willcox Co., Inc. Heavy Equipment Power Trains and Systems 584 The differential delivers power to the right and left tandem drives. See Figure 17-20. The tandem housing is attached to the differential housing. Each tandem’s housing pivots for- ward and rearward. Each tandem drive contains a planetary gear set, two drive sprockets, two chains, two driven sprockets, and two output wheel spindles. The planetary sun gear receives power from the differential’s side gear. The ring gear is stationary. The planetary carrier is splined to the two drive sprockets. Each drive sprocket has its own separate chain and propels a large wheel sprocket, which is splined to the output wheel spindle. The service brakes are located on the spindles. See Figure 17-21. Note The Doosan Da30 and Da40 articulated dump trucks have a tandem drive similar to that found on motor graders. However, the trucks use gears in place of the chain drives. Excavator Final Drives Excavator final drives receive their input from a hydraulic motor. The final drive often has two or three planetary gear sets. Each final drive on Caterpillar 330D, 336D, 374F, and 385C excavators has three planetary gear sets. The three planetary gear sets are incorpo- rated into a single final drive. See Figure 17-22. The machine has one final drive for the left track and one final drive for the right track. All three ring gears are part of the track drive sprocket housing. The hydraulic motor provides the input into the final drive by propelling the first stage sun gear (outboard sun gear). The ring gear is part of the sprocket housing and acts like a held member. The first reduction planetary carrier is driven in a slow forward torque multiplication. The first planetary carrier is coupled to the second stage (center) sun gear in the second reduction gear set, causing the sun gear to be the input member to the second gear set. The second ring gear is also part of the sprocket housing and acts like a held member, causing the second planetary carrier to drive in a slow forward torque multiplication. The second plan- etary carrier drives the last sun gear located in the third planetary gear set, causing the Goodheart-Willcox Publisher Figure 17-19. A tandem drive chain on each side of a skid steer propels the drive wheels. A hydrostatic motor drives two small sprockets that propel the two drive chains. The drive chains propel the driven sprockets, and the driven sprockets drive the skid steer’s axles. Philip McNew Figure 17-18. A skid steer hydrostatic drive motor. This Rexroth MCR hydraulic drive motor has integrated sprockets. It is used on Case SR200, SR210, SR220, SR250, SV250, SV280, SV300, New Holland L221, L225, L228, and L230 skid steers. The rear of the hydraulic motor has one Belleville washer spring that applies a multiple disc parking brake that must be hydraulically released for the skid steer to move. Drive sprockets Spring-applied parking brake Hydraulic motor