Gear reductions and speed multipliers: what they do in the coupling between pump and motor

Why rotation speed is a problem

The starting point is simple: pump and motor rarely "speak the same language" in terms of speed. A combustion engine can turn at several thousand revolutions per minute, a tractor power take off runs at a relatively low speed, an electric motor has its rated speed. The pump, for its part, performs at its best only within a well defined speed range.

If you connected the motor directly to the pump ignoring this difference, the pump would run outside its ideal range: too fast, with excessive wear and stress; too slow, with flow and pressure below expectations. The gear reduction or the multiplier is there precisely to prevent this.

Gear reduction and speed multiplier: two opposite functions

The two components solve the same problem in opposite directions.

The gear reduction lowers the speed: it is used when the motor turns faster than the pump requires. This is the typical case of petrol or diesel combustion engines, which run at high speeds and must be slowed down to suit the pump. The reduction ratio tells you by how much: a ratio such as 2:1, for example, halves the output speed.

The speed multiplier does the opposite: it increases the speed when the motor turns too slowly. This is the classic situation of the tractor power take off, which turns at a low speed and must be accelerated to bring the pump to the correct speed. Here too, the multiplication ratio states how much the speed is increased.

In both cases the mechanical principle is that of a pair of gears that turns the input speed into a different output speed.

Why connecting pump and motor is not enough

Neglecting the correct coupling has real consequences. A pump run too fast suffers accelerated wear of seals and moving parts, heats up and can cavitate; a pump run too slow does not reach the design flow and disappoints on performance. On top of that, a wrong coupling can overload the motor or the drive train.

Then there is the question of power: the gear reduction or multiplier must be sized to transmit the power absorbed by the pump, expressed in horsepower or kilowatts, without becoming the weak link in the chain itself. This is why it is not a simple fitting, but a mechanical component in its own right.

Types by motor

The choice of component depends first of all on the type of drive. For electric motors there are gear reductions designed for their rated speed and for that motor's shaft. For petrol and diesel combustion engines, robust gear reductions are used, able to handle high speeds and vibration. For drive through the tractor power take off, typical of agriculture, speed multipliers are used instead, bringing the low speed of the power take off up to the pump's required speed.

Completing the picture are the couplings themselves, such as flanges and the hydraulic motor connections, which mechanically join pump and motor when there is no need to change the speed.

A concrete example

Two very common situations make the logic clear. In the first, a pump is driven by a combustion engine turning at a high speed: here a gear reduction steps in, lowering the speed to the correct value for the pump and keeping it from running too fast. In the second, the same pump is connected to a tractor power take off turning at a low speed: now a speed multiplier is needed, raising the speed to bring the pump into its ideal range.

It is the exact same pump model, but with two opposite needs depending on the source of motion. This is why the right component depends not only on the pump, but on the whole combination of pump and motor: change the drive, and the device needed to couple them often changes too.

How to choose them

The choice rests on a few parameters, to be weighed together. The first is the ratio between the motor speed and the one the pump requires: from this follows whether you need a gear reduction or a multiplier, and with what ratio. The second is the power to be transmitted, which the component must handle with an adequate safety margin. The third is mechanical compatibility: the type and size of the shaft, the type of connection and the pump model to match.

One last recommendation: as with valves, gear reductions and multipliers should be chosen for the pump and motor pair, not separately. You can find the high pressure solutions among the gear reductions and multiplier gear boxes for plunger pumps, and the full ranges in the plunger pumps and diaphragm pumps sections; for the right match, our technicians can point you to the correct component based on motor, pump and running speed.