Versatility is one of the biggest advantages of hydraulics. It’s equally at home producing rotational motion as it is generating linear motion. Furthermore, today’s electrohydraulic controls make it a cinch to precisely control force, torque, speed direction, or any other dynamic parameter. Therefore, once a hydraulic drive has been specified for a machine, it only makes sense to tap into that hydraulic system for multiple functionality.
However, there’s a practical limit to the number and types of functions that should be served by a single pump. In some cases, you can add a second pump to the back of a primary pump — commonly called tandem, stack, or piggyback mounting. But tandem mounting generally is only practical when the secondary pump is substantially smaller than the primary one. So when you need to drive two or more pumps from a single internalcombustion engine, a pump drive box presents an effective solution.
A pump drive is attached to the flywheel end of an engine and drives a hydraulic pump or pumps. A modular pump drive encompasses multiple mounting points for attaching hydraulic pumps. It’s what makes an engine versatile because you have one power source. And with a pump drive — or multiple pump drives — you can split that power to run to many different locations.
To stack or not to stack
A common method for routing hydraulic power to multiple locations with multiple pumps is to use a single-output pump drive and stack multiple hydraulic pumps together. But before deciding to tandem mount multiple pumps, you should ask:
• Will the components that need to be powered be working simultaneously? If so, can the engine provide the required torque?
• Does the vehicle impose any space constraints that would preclude stacking pump drives?
When one pump is stacked behind another, all the power for the pump stack in total must be transmitted through the primary pump input shaft. Often that shaft lacks the torque capacity to drive all pumps simultaneously. Another limitation comes from the vehicle, which sometimes doesn’t have the length available to stack the pumps. Also, Tier 3 and Interim Tier 4 emissions-certified engines are introducing some restraints on the available room in the engine compartment. This means some creativity and flexibility in the packaging of components is sometimes needed.
Whether or not it’s feasible to stack pumps on a single pump drive output usually depends on the torque capacity of the primary pump’s splined input shaft. Splined shafts are usually sized specifically for a single pump, and the load is increased when additional pumps are tandem mounted to the primary pump. If those pumps will never be energized at the same time as the primary pump, then there’s no issue. But if two or more stacked pumps come on simultaneously, then there’s probably not enough capacity in that primary shaft to transmit power to both secondary pumps. In any case, the benefits of a modular pump drive should now become apparent.