Heavy duty hydraulic motors
Original article date: February 2000
With its acquisition of Hydrostatic Transmissions, PSS is now offering high torque, low speed radial piston motors, integrated into its own range
Now being supplied by Steering and Hydraulics Division of North Walsham, Norfolk based PSS, is the HTL range of rotating shaft hydraulic motors. The principle of operation of these motors is based on high pressure oil fed to the crankshaft via the distributor housed in the end cover casing.
This cylinder block carries five radially disposed pistons. The base of each cylinder bore has a port which connects with the machined slots in the crankshaft eccentric.
High pressure oil enters the base of the cylinder bore and exerts a force through the piston nearest the centre line of the motor and attempts to give the piston an outward motion as a result of the imbalance in the areas of the two piston ends.
Direct transmission
The outer face of each piston is seated against the flat machined thrust face in the crankcase, so the reaction to this force is a thrust transmitted direct to the eccentric by a high pressure column of oil.
An Oldham plate and dogs maintain the correct angular relationship between the cylinder block and crankcase. Because there are either two or three chambers exposed to high pressure at all times, total torque output is the vector sum of the torque outputs developed by the multiple columns of fluid. This results in very smooth rotation, and virtually constant torque even at very low speeds. The design also offers very high starting torques.
When used as a wheel motor, the shaft is held stationary and acts as an axle, while the rotating housing serves as the wheel hub. High pressure fittings connect directly to the shaft end. Features of this compact wheel motor include:
- Single-ended shaft designs allowing for cantilever-type mountings
- Elimination of axles, drive shafts, universals and gearbox transmissions
- Lower centre of gravity, allowing for lower vehicular profiles, yet with equivalent or better ground clearance
- Operates in adverse environments without special protection
- Direct internal drive
- Freewheeling capability, allowing vehicle speed variation in multi-motor applications.
One use for this compact wheel motor is to power a wheel for a greenkeeping machine, and it can also be used as a direct internal drive for winches, conveyors and other rotary drum-type applications.
Of particular interest to past and prospective users of the HTL products will be the re-manufacture, repair and spares service of PSS.
|
The essentials for low speed radial piston motor operation 1. Mounting: Motors can be mounted in any plane, preferably to a flat machined surface. In addition, torque arm mounting may be employed with some units. 2. Fluid: A good quality mineral hydraulic fluid should be used compatible with operating temperature and viscosity. Operating with high water base and synthetic fluids is acceptable subject to approval. 3. Temperature: The normal operating temperature is +20 to +50degC. Extremes of temperature are -20 to +100degC. 4. Viscosity: Best performance is generally achieved with relatively high viscosity.
5. Filtration: A filtration efficiency of b10=75 and a fluid cleanliness in accordance with ISO 4406 contaminant code is recommended. 6. Case Drain: To maintain a full motor crankcase, it must be filled with oil before starting and the drain port independently connected to the reservoir. 7. Freewheeling: The motor can be freewheeled by connecting both the main ports directly to tank, with no pressure. At the same time, the crankcase drain port must be pressurised to 1.4bar (20psi). Under these conditions, the pistons are withdrawn and no pumping action occurs, allowing minimal resistance to rotation. The case maximum pressure must not be exceeded or the shaft seal will be damaged. 8. Charge pressure: Positive pressure must always be maintained at both ports during motor and pump modes of operation. |
- PSS
February 2000