Dynamometer

If you've got drawbar pull and speed you've got power, no need to instrument band brake

Hello Noel,

my idea to build a test stand while standing is to brake the wheels on which the loco is standing . All the wheels of the testbench have to be coupled and braked by an electric brake . This can be made with a large dc motor as variable brake or with a brushless motors. Then in the bench can be measured with a drawbar as it is possible during driving with a waggon. The loco have to be fixed anyway on the test bench. On the front there have to be a fixture with lot of play and no contact of the front ,but for emergency reason it have to fix the setup. The same is on the rear side necessary.

Werner

Harking back MANY years, my mind immediately recalled using bridge connected strain gauges to compensate for any temperature, voltage changes during running..

So just musing.

With known speed and drawbar pull, the power can be calculated (That's how the full scale railways did it until the advent of Swindon's "Home Trainer", and eventually the Vichy rolling road.) They measured drawbar pull using the extension / compression of springs of known rate.

A strain gauge set up would be calibrated by hanging weights on the drawbar whilst suspended vertically, and noting the readings., effectively measuring the rate of the drawbar as a spring..

The energy gained by being pushed downhill will become "visible" as momentum (Increased kinetic energy available in excess of that required to overcome friction in the trailing load, in addition to that being provided by the cylinders, / prime mover ) resulting in greater speed at the bottom of the incline, and along level track, or the start of a rising gradient.

If the drawbar load becomes negative, the loco is not producing tractive effort. Its output is being used to overcome friction and any aerodynamic drag (At 10 mph???? ).supplemented gravitational effects, assuming that the controls remain untouched.

Howard

Werner and Howard, thank you for your replies. I'm a long way from the theories of dyno testing yet ! The first problem is to get the power from the wheels to somewhere it can be measured without much slip or large losses in friction Etc . Steel wheels on small steel rollers seems to invite slip ! Nobody has commented on the idea of toothed rollers carried on bearings and a timing belt of suitable size/length driving the power absorbing device. Each driving/coupled wheel would sit on the belt, being supported from below by a toothed roller under the C/L of the wheel. with both wheel and belt clean and dry and the belt deforming under the weight of the loco one would hope to get much better adhesion by vertue of a greater contact area - any thoughts. Using a suitable belt and bearing in mind the modest power involved it should work ??? Noel

Noel,

to use the tooth belt wheels as suport for the loco wheels is a good idea. There is more adhesion to prevent the wheels from slipping. Additional it is easy to connect the brake alternator. As brake alternator I would use DC motors from a car scrapyard and there i would collect the motors of the car radiator. They have normally at 12 Volt 30 Amp current consumption. With an estimated efficiency of 70 % you have about 250 Watt brake power of one motor and it is clear you need more than one motor. A second good source is a motor of an electric steering they are in a very good quality. The best source is a defect electric steering column. There are DC and brushless motors available. And they are in a range of 12 V 60 A with an efficiency of more than 80%

If you measure with a dyno drawbar a small wheel slip is not a problem.

Werner