Capacity says nothing about performance

From Horsepower to Watts to Newton Metres

From Horsepower to Watts to Newton Metres

Did you know that when galloping a horse has twenty horsepower, but when walking normally only two thirds of that? Horses would be amazed! In the case of cars, horsepower – with which we have long been familiar – has been replaced by kilowatts. Naturally we still look for the HP figure. What engine engineers are interested in, on the other hand, are Newton metres. Why is that so? And why are Newton metres so important when it comes to pneumatic motors?

Torque is key
The speed of a sports car versus the pulling power of a truck Appliance manufacturers often indicate performance in watts.

The figure they show here represents the maximum capacity. In reality, this maximum output is only achieved in ideal conditions. Yet, only a limited amount of meaningful information about performance and the degree of efficiency under working conditions can be gleaned from the wattage details. A machine’s bite is based above all on torque and revolutions per minute. The greater its torque, the stronger a machine pulls. If it does this from the start, that is even at low revs, then you’re dealing with a powerful motor. That’s also the reason why a high torque 600HP truck has greater traction than a 600HP sports car. You have to decide what you want: to whiz along or to work.

Why Newton metres?
Force equals mass times acceleration Torque is measured in Newton metres (Nm).

Torque of 1 Newton metre is produced when a force of 1 Newton is exerted on a fulcrum using a lever length of one metre. 1 Newton is defined as follows: 1 Newton = 1kg x m/s². This formula means that if you move 1 kilogram by 1 metre per second, then you produce 1 Newton. In physics force is thus understood as mass times acceleration. Based on the international system of units (SI – Système International d’Unités), force is stated today in Newtons, after Isaac Newton (1643-1727), the first man to describe the connection between force, mass and acceleration.

Sewer rehabilitation using robots
Lightweight compressed air hoses are important

What is key for compressed air motors is the force – measured in Newton metres – that they develop from the lower revs range up. A high wattage figure, on the other hand, means that they use a lot of compressed air. That, however, leads precisely to the compressed air hose for powering the robot tools needing to have a large diameter. Milling work using robotics underground requires lightweight power supply systems. Heavy hoses restrict robots in their freedom of movement. Inside sewers long hose combinations of up to 50m and more and needed. However, as the hose gets longer the operating pressure drops. Pneumatic motors with high air consumption and therefore high wattage are not a good solution for robotics operations. What is important for a compressed air motor is the lowest possible air consumption. The key performance indicator we’re interested in is not watts, but force in Newton metres.

Earthly engineering arts
Pneumatic motors with high torque thanks to epicyclic gearing

Nowadays, pneumatic motors are fitted with an epicyclic gear set. Epicyclic gears permit a slim design and stabilise the torque under load. Pneumatic motors without gears lose power quickly during the working cycle, sometimes by up to 70%. The following table illustrates the dependency on air pressure and shows how relative output (wattage) is: pneumatic motors are designed for an operating pressure of 6 bar. A fall of air pressure from 6 bar, to 5 and then 3 bar leads to a steep drop in output to down to 75% and further down to 36%. The nominal torque (maximum torque), on the other hand, behaves in a much more stable manner. If pressure falls by 2 bar to a working pressure of 3 bar, a 1600-watt motor no longer outputs any more than 576 watts (36%). Wattage is thus a meaningful figure only under ideal conditions. The key performance indicator is the Newton metre figure.

Published in RO-KA-TECH Journal January 2007

Schwalm Robotic GmbH Industriestraße 16 36251 Bad Hersfeld-Asbach Germany Telefon: +49 6621 79578-0 Telefax: +49 6621 79578-11