How Does a Hydraulic Pump Drive Work?

A hydraulic pump drive looks like a simple gearbox, and in many ways it is. Understanding how power moves through it makes it much easier to choose the right model, troubleshoot a problem, or talk through an application with a supplier. This article walks through how a pump drive works, from the engine connection to the hydraulic pumps.

The Basic Power Flow

Power moves through a pump drive in a straight line:

1. The prime mover, usually an industrial diesel engine or electric motor, turns the pump drive's input.
2. The input turns the gear train inside the pump drive housing.
3. The gear train turns an output gear at each pump pad.
4. Each output turns a hydraulic pump, which creates flow and pressure for a hydraulic circuit.

Input and Output

The input is everything that connects the pump drive to the prime mover. The most common input is a drive plate or coupling that bolts to the engine flywheel, with the pump drive housing bolting to the engine flywheel housing. The outputs are the pump pads, the mounting locations where hydraulic pumps attach. Each pad has a flange and a splined or keyed shaft connection sized to match a standard hydraulic pump.

Inside the Gear Train

In a gear-driven pump drive, the input shaft turns a drive gear. That gear meshes directly or through idler gears with a gear at each output pad. The size relationship between the drive gear and each output gear sets the speed of that pad. Simple pump mounts skip the gear train entirely and connect a single pump to the engine through a coupling, so the pump turns at engine speed.

How Ratios Work

A pump drive ratio compares input speed to output speed. A 1:1 ratio turns the pumps at engine speed. A speed-reducing ratio slows the pumps down, and a speed-increasing ratio speeds them up.

For example, if an engine runs at 1,800 RPM and the hydraulic pumps perform best at 2,250 RPM, a speed-increasing ratio of 1:1.25 brings the pumps to the right speed without changing engine RPM. Many pump drive models are offered in ten or more ratio options.

Gearing also affects direction of rotation. Depending on the gear arrangement, a pad may turn in the same direction as the engine or the opposite direction, so the rotation of each pump must be confirmed during selection.

Torque and Power Ratings

Pump drives carry two kinds of ratings. The input rating covers the total power or torque the pump drive can accept from the engine. Each pad also has its own rating, which limits the load any single pump can place on it. The combined load of all pumps must stay within the input rating, and each pump must stay within its pad rating.

Lubrication and Cooling

The gears and bearings in a pump drive run in an oil-filled housing. Moving gears carry oil to the bearings and gear teeth, and the housing sheds heat to the surrounding air. Units running at high power or in continuous duty may need an oil cooler to keep temperatures in range. Regular oil changes and level checks are a basic part of pump drive maintenance.

Disconnecting the Load

Some applications need the hydraulic pumps to stop while the engine keeps running. There are two common ways to do this:

• Input clutch: A clutch between the engine and the pump drive disconnects the entire pump drive and every pump on it. This is useful for starting a cold engine without a full load.
• Live PTO: An air- or hydraulically-actuated clutch at an individual pad disconnects one pump while the others keep running. This saves energy and extends component life when a function is not in use.

Connecting to the Engine

Most industrial engines follow SAE standards for flywheel and flywheel housing sizes. Housing sizes run from SAE 6 (smallest) to SAE 00 (largest), and common flywheel sizes range from 6.5 to 21 inches. A pump drive's input must match both the engine's housing size and its flywheel size, so measuring them correctly is one of the first steps in any pump drive project.