How Do I Control RPM?

How Do I Control the RPM?

RPM is a function of coolant flow. Depending on the application, the head, and the shank style there are various way to control the spindle speed.

In general it is less important to control the speed of a coolant driven tool than it is an electrically driven tool. Electric tools will try to maintain a constant speed no matter the load it sees. The nature of a hydraulic drive is to run faster with no load and slow down as the load increases. The torque will also increase as needed. This is because as the load restricts the coolant flow the coolant pressure will climb.

As the load decreases the flow (and RPM) will rise and the pressure will decrease. Unlike an electric drive you have the most power when you most need it.

While it is less important to precisely control spindle speed we do need to get it in the right “ballpark”, because “power” is a result of speed × torque. If we want to make significant reductions in the RPM it is best to either hydraulically or mechanically gear the head to the desired lower speed. By gearing the speed down rather than just restricting the coolant flow we get an inverse increase in torque to compensate for the lost RPM and our power will stay the same.


If we have 1000 RPM and a torque of 10 our power = 10,000 (1000 × 10 = 10,000)

We can lower our RPM to 500 by restricting the coolant flow by half but while restricting the coolant flow wil result in higher coolant pressure at no load it would not increase the coolant pressure at maximum load. So when calculating our power we would still use a torque of 10.

500 × 10 = 5,000

As you can see our power is now 1/2 what it was.

If instead we put a motor twice as big the result would be twice the torque at 1/2 the speed, or 500 RPM at a torque of 20.

500 × 20 = 10,000 so we have reduced our speed to 500 without a loss of power.

We do have optional motors available as well as a 5:1 planetary gearbox that fits between the motor and the head. If you want to explore what is most appropriate in your specific application give us a call and we can explain all your options.

More minor speed adjustments can then be accomplished by regulating coolant flow. Some coolant pump manufacturers now offer variable pressure/flow pumps. Our heads also have flow controls built into them where possible.

On straight shank tools the coolant enters the tool from the end of the shank. The hole is tapped ¼ NPT. An orifice can be drilled in a pipe plug to restrict coolant flow. The same can be done on retention knobs.

Din B holders usually have flow controls on the flange ports.

Orifice Charts are available courtesy of Chipblaster.

Standard motors will run 900 RPM per Gallon of flow. Standard motors will run 238 RPM per litre of flow.

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