ARTICLE
over another. Usually, the design envelope that the OEM engineer has to
physically fit the pump within eliminates some options or maybe even all
but one. However, it is important to understand the pros and cons of each
type of input.
On-highway equipment
On-highway applications such as concrete mixer trucks, truck mounted
cranes, and dump beds rely on traditional mechanical transmissions to
move the vehicle. For these vehicles, there are a few options for providing
the rotational power for on-board hydraulic pumps. The hydraulic pump is
generally mounted on the side of the vehicle’s gearbox integrated power
take-off units.
A power take-off (PTO) is any of several methods for taking power from
a power source, such as a running engine, and transmitting it to an
application, such as an attached implement. Most commonly, it is a system
comprising of a splined output shaft designed so that a pump drive shaft
can be easily connected and disconnected. The PTOs are engaged and
disengaged by means of the truck’s pneumatic system.
For these types of inputs, there is often little space between the PTO and
the transmission axle; therefore, the size and shape of the pump must be
considered. A large displacement pump may be too wide or long for using
this input area (because of interference with the axle) so it is important to
research the pump manufacturer’s options for mounting requirements.
Figure 1. For on-highway applications the hydraulic
pump is generally mounted on the side of the vehicle’s
gearbox integrated PTO with tight space parameters
around the axle. Bent-axis pumps contain pistons
mounted at an angle offering a compact solution that
still provides a wide range of displacement.
Many pump manufacturers, such as Bosch Rexroth, offer pump mounting
flange options that are compact in space or bent-axis pumps (Figure 1) that
would offer better clearance around the axle. Bent-axis pumps are similar
to bent-axis motors in that the piston pump contains pistons mounted at
an angle to a drive shaft, which rotates the pistons. This angle, typically
around 25 degrees, allows for a compact mounting area yet provides a
wide range of displacement.
Auxiliary engines (in addition to the main vehicle engine) are often seen
in truck mounted applications when full engine horsepower and torque
are required. Applications such as large vacuum pumps typically use this
method. In these cases, there can be multiple pumps on a multi-pad
gearbox that is directly driven by the auxiliary engine’s output shaft. This
type of configuration allows for multiple pumps, as well as potentially
greater flexibility for pump size considerations. In this situation, it is
important to make sure that there is adequate horsepower available to
the hydraulic pump(s) for the intended performance of the function being
controlled.
Off-highway equipment
Figure 2. Usually found in machines that
use the hydraulics system as their only
means to perform work, like excavators and
feller bunchers, engine flywheel mounts for
pumps allow a direct coupling to the diesel
engine and offer installation space savings
for applications.
In comparing on and off-highway equipment, it is best to understand the
base design principles for each type of equipment. On-Highway systems
are usually based on an existing vehicle drive chassis. Off-Highway
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