Description and Operation 4000 SRM 736
A. PHASE 1 B. PHASE 2
1. MAIN LIFT CYLINDER (2)
2. OUTER LIFT CHAIN
3. FREE-LIFT CYLINDER
4. INNER LIFT CHAIN
5. INNER WELDMENT
6. INTERMEDIATE WELDMENT
7. OUTER WELDMENT
8. FROM MAIN CONTROL VALVE
9. EXTERNAL LOWERING CONTROL VALVE
10. INTERNAL LOWERING CONTROL VALVE
Figure 4. Three-Stage Mast With Full Free-Lift
Two main lift chains fasten at one end near the top
of the outer weldment. The lift chains then go over
sheaves at the top of the intermediate weldment and
fasten at the bottom of the inner weldment. Free-
lift chains are connected to a mount behind the free-
lift cylinder. Chains then go over the sheaves and
connect to the carriage.
Three lift cylinders are connected by hoses. To extend
mast, oil from main control valve flows to all cylin-
ders at the same time. Free-lift cylinder raises first
because it lifts the least amount of weight. Free-lift
cylinder raises the carriage to the top of the inner
weldment. After free-lift cylinder reaches the end of
its stroke, the main lift cylinders begin to extend. As
main lift cylinders extend intermediate weldment,
inner weldment is extended by lift chains.
During lowering, main lift cylinders lower first be-
cause they have a greater load. After the main lift
cylinders have retracted, free-lift cylinder lowers. All
oil from lift cylinders flows through the lowering con-
trol valves to the hydraulic tank.
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