Fig. 1
The CS56 soil compactor uses Caterpillar’s pod-style vibratory system to deliver high compactive force while offering dual amplitude selection and innovative design ensure precise performance.

Hydraulic Motors Transmit High Torque for Machine Motion

Hydraulic cylinders usually answer the call for generating high torque in linear drives. But when rotational power is needed, it’s hard to beat hydraulic motors for high torque with precise speed control.

Whether heavy equipment needs rotational motion for several operations or only one, hydraulic motors often make sense because they can generate high torque from a compact package. But the ease of controlling speed, torque, and direction using simple hydraulic valves makes hydraulic motors versatile, as well.

This report explores examples of different that machines that capitalize on the many advantages of hydraulics to make them leaders within their respective fields.

Hydrostatic Vibration Enhances Compaction

There’s more to surface compactors than what meets the eye. They appear to have a large, heavy drum that rolls across the surface of loose soil or fresh asphalt to compress it down into a flat surface. But that’s not all: They also contain a vibration mechanism that drives an eccentric weight to use force from angular acceleration to apply additional downward force.

The principal works much like compacting soil with your feet. If you stand or walk slowly on loose soil, you’ll compact it with your body’s weight. But if you jump, and stomp on the soil with your feet as you land, the combined force from your weight and leg action will produce much more effective compaction than your weight alone.

In the Model CS56 soil compactor from Caterpillar Paving Products, force from angular acceleration is produced by a hydraulic motor driving a pair of eccentric weights inside the drum. It uses a 30-Hz, dual-amplitude vibratory system to help it deliver optimum compacting force. Changeable from the operator’s station, either of two vibratory amplitudes can be selected to best match the soil characteristics. Centrifugal force in high amplitude is 28,000 lb and drops to 13,800 lb in low amplitude. These medium-duty soil compactors with 66-in. wide drums have estimated production rates up to 500 cubic yards per hour.

The weights are installed on a vibratory shaft and supported by heavy-duty bearings in an oil-bath compartment inside the drum. A hydraulic motor in a dedicated circuit rotates the shaft. Each weight has a sealed, off-center interior cavity that is partially filled with steel shot. The rotating mass of off-center steel shot produces axial vibration that combines with the weight of the drum for high compaction force.

Hydraulic motor drives vibratory shaft carrying pair of eccentric weights in oil-bath compartment inside the compactor’s drum.

The motor typically rotates at a nominal 1,800 rpm to produce vibration of 31.9 Hz at a controlled amplitude of 0.9 or 1.8 mm. (An optional, variable-displacement pump provides adjustable frequency from 23.3 to 31.9 Hz.) The geometry of the weights produces different amplitudes of vibration based on the direction of rotation. Simply reversing the direction of the hydraulic motor moves the steel shot into another position within the cavity, changing the weight’s center of gravity. The result is high centrifugal force (282 kN) in one direction, and low (141 kN) in the other. The result is simple, dependable amplitude selection with no chance of introducing metal flakes into the bearing lube-oil reservoir.

The CS56 has hydrostatic-drive arrangement powered by tandem variable-displacement piston pumps. One pump supplies flow to the drum drive motor; the other separates flow to ts56he rear-axle drive motor. The rear-wheel-drive powertrain also includes a gear reducer, no-spin differential, and planetary wheel ends on the axle to maximize rear-wheel traction. A balance valve can interconnect the two traction circuits. When necessary, it shifts to pass flow to allow for slight variations in the rolling radii due to changes in the surface density.

This configuration provides maximum torque and full-time tractive power to both drive motors, regardless of footing or rolling resistance. Should either or front or rear drive lose traction, hydraulic flow to the non-spinning motor ensures continuous tractive effort. The soil compactor’s maximum travel speed—in forward and reverse—is 11.7 km/hr (7.2 mph). Each propulsion motor has a flushing valve that responds to return-line pressure and ports part of the return flow through an oil cooler. The closed-loop hydrostatic drive system also provides dynamic braking during operation.

In the power-assist steering system, two 76-mm (3-in.) bore, double-acting cylinders—supplied by a gear pump mounted on the engine’s accessory drive—provide smooth, low-effort handling. The steering system receives the power it needs regardless of the status of other hydraulic functions. The resulting mobility combines with high tractive effort to allow the CS56 to perform under difficult conditions, such as compacting loose soil on slopes and climbing steep ramps in trench-filling applications.

Operator’s Platform

Operators appreciate the design of the operator’s platform with grouped, low-effort controls. All propel and vibratory controls are located to the operator’s right for one-handed actuation. There is unrestricted visibility to the drum edges and the sloping engine compartment improves visibility to the rear. Four rubber shock mounts isolate the operator’s platform from vibration.

For serviceability, the operator’s platform tilts and locks open to provide easy access to valve blocks and other components in the hydraulic system. Grouped quick-connect pressure couplings are standard, and O-ring face seal hydraulic fittings are used throughout to eliminate leakage.

Check out the video below to see the CS 56 in action.

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