Precautions When Using Hydraulic Motors As Pumps

First and foremost, since you have purchased a hydraulic motor, it should be used as a motor. Pumps and motors are fundamentally different in design and are not fully interchangeable. In practice, only gear pumps can function as hydraulic motors, while piston pumps and vane pumps are entirely unsuitable for motor applications. A motor is a motor, and a pump is a pump. To use an automotive analogy: we would never expect a car starter to continue operating as an alternator after the engine has started.

Two Scenarios for Using Hydraulic Motors as Pumps:

1. Pressure Control in Inertial Flywheel Systems
   When a hydraulic motor drives a large flywheel, abruptly cutting off the oil supply to the motor can cause a dangerous pressure spike (theoretically infinite) due to the flywheel’s inertia, potentially damaging components. This phenomenon is analogous to the back electromotive force (EMF) generated when disconnecting inductive components (e.g., motors or solenoid valves) in electrical circuits—stored energy creates extremely high voltages that can destroy sensitive electronics if unmanaged.

   Another example is hydrostatic drive systems (e.g., in lawn mowers). When the pump stops supplying oil, we want the equipment to decelerate gradually rather than stop abruptly. Here, the motor must transition into pump mode to absorb energy progressively, enabling smooth deceleration.

   Solution: Install a safety valve in the system to limit maximum pressure. Excess pressure can be stored in an accumulator or dissipated as heat via a relief valve.

1. Inertial Flywheel System

2. Multi-Power Source Switching Systems (Rare Cases)
   A recent truck drive system I designed exemplifies this scenario: three independent power sources drive the truck via a roller chain.

• Phase 1: A hydraulic cylinder pushes the truck into motion.

• Phase 2: A precision electric servo motor positions the truck for machining.

• Phase 3: A hydraulic motor resets the truck to restart the cycle.
  Although the hydraulic motor operates for <10% of the total cycle time, it remains connected throughout the process. Thus, it effectively acts as a pump for 90% of the time.

Cavitation Phenomenon
Cavitation typically occurs when a pump draws oil from a reservoir with insufficient supply. The pump attempts to pull oil via vacuum, but due to oil’s incompressibility, localized high temperatures vaporize the oil, creating bubbles. These bubbles collapse violently in high-pressure zones, causing shockwaves and damaging the pump.

Causes of Cavitation:

• Insufficient oil volume in the reservoir

• Clogged suction line filter

• Blocked suction strainer

• Clogged or missing breather

• Excessively long suction lines

• Undersized suction line diameter

• Pump installed above the reservoir oil level (lacking self-priming capability)

Preventive Measures:

• Regularly inspect filters, breathers, and oil levels (recommended as part of maintenance schedules).

• Ensure the pump inlet has a positive pressure head (reservoir oil level above the pump suction port) during design, and minimize suction line pressure drop. Ideal suction line velocity should be ≤1.5 m/s, with a pressure drop ≤6.9 kPa (determine pipe diameter using hose calculators).

Special Note: Even short-term use of a pump as a motor requires cavitation prevention. If space constraints necessitate using a motor as a pump, larger pipe diameters are often required to compensate for pressure losses in long suction lines.

Motor Efficiency Concerns
Since motors are not optimized for pump operation, their efficiency is typically 10%-20% lower than rated values (varies with pressure and flow). Inefficient operation generates excess heat, requiring dissipation via high-pressure heat exchangers or additional return lines. If a motor must operate long-term as a pump, a dedicated cooling system is mandatory.