The limited space available for a single PTO from a standard engine can complicate hydraulic-pump installation. Depending on the desired pump displacement, the space required to install the pump may prevent easy mounting to the engine PTO. As a result, the manufacturer may need to use spacers or other costly add-ons to mount larger displacement pumps. Once again, it’s best to consult the pump supplier regarding space-saving options for larger displacements.

An example of a potential solution comes in the form of Bosch Rexroth’s A10VNO medium-pressure open-loop pump. The compact unit can fit into small spaces that require high displacement. Another alternative is to use two smaller pumps in a tandem/through-drive package. When combined, they’re smaller in diameter than a single, larger pump, but considerably longer.

To achieve greater flexibility in multipump applications, however, designers may consider multipad PTOs. If multiple pumps need to operate at different input speeds, a multipad PTO will provide different output speeds by changing each pad’s internal gear ratios.

Multiple pumps also can be driven by a pump’s through-drive. Most axial-piston pumps offer a through-drive option that contains a rear output shaft for driving a second pump. To accommodate different mounting flanges, through drives come in a variety of sizes — such as SAE, A, B, C, or other standards. This method can create many different combinations, but you must consider the available maximum torque levels each pump or through-drive can provide to each subsequently mounted pump. Other considerations include maximum limitations of the pump stack’s bending moments, weights, length (long assemblies may require mechanical support at the back end) and available torque limitations at each pad. 

Other Factors Influence Input Choice

Even with the cases mentioned, other elements of the machine’s design should be considered when determining the hydraulic pump’s input method. For one, there’s the impact of Tier IV final emissions on machine design. Additional engine-exhaust-treatment components will occupy more engine compartment space on mobile equipment. This presents new challenges because more pumps with higher power density will be required due to space limitations. These power-dense pumps may operate at higher pressures, which could alter how the pump should be driven within the system.

Moreover, the upcoming Tier IV final engines generate a more demanding torque-ripple effect. Pumps used on direct engine drives and PTOs will be subject to higher angular acceleration fluctuation of their input shaft. Most pump manufacturers publish angular acceleration limits for input speeds.

Jay Edwards is Sales Manager, Agri/Forestry Machines, Bosch Rexroth Corp., Fountain Inn, S.C. Contact him at (864) 228-3011, or visit