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Pump selection for my commercial vehicle - What does it depend on?

To determine the right pump for my (commercial vehicle) application, there are several points to consider. First, we need to know whether the application requires a fixed displacement pump or a variable displacement pump. The size of the pump is defined by the amount of oil and pressure required. Whether the selected pump can be used is determined by the performance data such as gear ratio, direction of rotation, maximum torque, usable continuous power, as well as the stall torque of the power take-off (hereafter referred to as PTO). It is also important to know whether the power take-off is

is switchable or not.

Some explanations and definitions beforehand

Variants of PTOs

In trucks, a distinction is made between engine PTOs and transmission PTOs. PTOs that are mounted in the drive train (e.g. distributor gearbox) are less common. Engine PTOs are clutch-independent and often cannot be switched. During operation, the pump always rotates at the maximum permissible speed. Ideally, a variable displacement pump is mounted so that only as much oil as needed is pumped. If a fixed displacement pump is used, a circulation/bypass valve must be installed. Optionally, a switchable clutch can be used (Possible application: hook implement) Gearbox PTO's are clutch-dependent and are switchable, depending on the arrangement, two different gear ratios are possible, half-gear low or high (Possible application: tipper).

Tilting moment / bending moment

Centre of gravity of the pump acting on the mounting flange. If this is exceeded, additional support for the pump must be provided.

Mounting flange/shaft

  1. The "UNI" version is mainly found on delivery vans.

  2. For heavy commercial vehicles, the type DIN ISO 7653 (Ø 80 mm) with shaft ISO14 DIN 5462 8-32-36" is predominantly used.

  3. The SAE types are less common in commercial vehicles.

Volumetric efficiency: ηvo 90-95%.

Hydr.-mech. efficiency: ηmh 90-95%

Total efficiency: ηt = ηvol x ηmh => 80-90%

Types of pumps

Gear pumps for truck power take-off are particularly suitable for smaller volume flows and pressures up to 200 bar. They are often used for steering.

For axial piston pumps, we distinguish between fixed displacement pumps (XPi) and variable displacement pumps (TXV).

Fixed displacement pumps (XPi) in the familiar inclined axis design are suitable for many applications up to a maximum of 380 bar operating pressure (peak 420 bar). They are available from 12-130 cm³/rev. Optionally, by-pass valves are available as accessories for power take-offs that cannot be switched off.

Variable displacement pumps (TXV) with load sensing controller are often used in combination with truck loading cranes and wherever the oil quantity needs to be variably controlled. These are available from 40-150 cm³/rev and pressures up to 400 bar (peak 420 bar). Torque control is also available as an option. This allows a large delivery capacity to be realised even with weaker PTOs.

Practical example

Our customer would like to realise a system with a variable displacement pump with a litre capacity of 125 l/min at a maximum operating pressure of 350 bar. We assume a speed of 1000 rpm of the pump (corresponds to 840 rpm motor). First we need to know the possible drive power, direction of rotation, transmission ratio, breakdown torque and the maximum usable torque. We take this from the technical specifications of the PTO, (see picture Scania ED160), this is an engine power take-off.

Attention: The direction of rotation of the PTO is clockwise, this means the pump must be counterclockwise!

We calculate the required delivery volume and the torque of the pump as follows:

Based on the 2nd calculation, the permissible torque is exceeded by 170 Nm.

We calculate the possible displacement of the pump via the pressure and the possible torque of the PTO:

Alternatively, the possible sip volume can be seen in the pump manufacturer's diagram:

The following three solutions are possible:

(No continuous power, only calculated via torque).

The desired delivery rate of 125 l/min can be maintained up to around 275 bar with variant 3. From this value, the power regulator reduces the delivery volume until the permissible torque of 600 Nm is no longer exceeded. At the maximum operating pressure of 350 bar, around 98 l/min are available.

We will be happy to assist you in selecting the right hydraulic pump. Benefit from our experience and, of course, from our extensive delivery programme.


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