Monthly Archives: November 2014

11-18-14 draw bar force rev 11-20-14

In the testing that has been done to date, the largest question in in the draw bar force measurement. It seems to fluctuate more than can be easily explained.

The plan has always intended to use multiple techniques to be able to verify the draw bar pull. The first two involve using two force measuring instruments.

The first is from Micro Mark and the other is a trigger pull measuring device. The cross checks with these two have generally been in agreement. The problem is that in most cases the wheels are slipping at the voltage when the measurement is made. The drawbar force in this environment seems to inconsistently represent the actual pull force of the locomotive.

Two other techniques are in the wings ready or close to being ready to use to facilitate the measurement of the draw bar force. The first involves the use of a pulley and weights. The second involves a multiple car simulation vehicle. The later is in process and will be discussed in a later post.

The former is intended to get a direct measurement of tractive effort versus the velocity for any engine. The pulley is installed at one end of the test track. A twine is draped around the pulley. One end is connected the test locomotive and the other is connected to a weight hook. The weight hook hangs down from the pulley.

With a given weight, at a particular voltage the engine will raise the weight at specific speed. The weight is the tractive force minus the friction of the pulley. There is a weight value that will stop the engine from moving. That should be the maximum draw bar force for the input voltage. As the voltage increases, this force will also likely increase. At some voltage input, the wheels will start to slip. Depending on the locomotive, the force may or may not increase at higher voltage input. Classically the force at this wheel slip condition is quoted as the draw bar force.

Some how this seems like a multiple combination solution. The force at wheel slip at a low velocity may be the maximum. The question is will it always be the maximum?

As wheel slip is increased due to increased voltage beyond this point, may cause the force to fall off. With some tests that were performed today, it appears that this reduction is occurring. This brings into question the value of the gauge measurements.

In the near term, the weight and pulley needs to be implemented and some of the engines tested to this point need to be examined using this technique. This will help with the understanding of the measured draw bar force measurements.

(Revision) Because of needs to move the recently tested engines along, an interim attempt to get better numbers from the instruments. Eight engines were reexamined looking for the maximum pulling force at what ever voltage setting it occurs. In addition, the draw bar force at 12 volts in addition to the maximum force. The maximum force was found between 11.5 and 16 volts.

The level of these forces are generally greater than the value measured at 16 volts. Not in every case. The engines designed in the DC era tend to have a larger value at or near 16 volts. The engines that were designed for DCC the peak force occurs at or slightly below 12 volts. This is not surprising since that is near the maximum the decoder will deliver to the motor.

The DCC engines pull force is significantly lower than the DC engines. This is not surprising. The quickest way to a quiet motor is to reduce its power. This reduces the current levels the decoder will see. The problem is that the torque is also lower. In some cases, a reduction by more than fifty percent.
The pulley weight technique is still high on the list, but a satisfactory interim technique in now part of the testing activity.