Spring force can have a dramatic effect on the life and performance of carbon brushes because it has a strong influence on mechanical and electrical wear. Many brush performance issues are resolved by the spring rather than the carbon grade, and the consequences of ignoring spring force can be inefficient at best and catastrophic at worst.  Measuring spring force allows you to detect issues before any damage is done to the brushes or motor.

Issues with Spring Force Can Cause:

  • Rapid brush wear.
  • Threading of brushes and, in extreme conditions, even the contact surface.
  • Fraying of wires resulting in loss of current carrying capacity. If not addressed, the wire can break, causing current to follow unintended paths leading to other issues or failure.
  • High-intensity arcing that could cause grooves in the brush track.
  • Increased dusting with metallic dust from the contact surface that is more conductive and can cause flashover.
  • Other commutator issues including streaking, bar edge burning, and copper drag.

Three Steps to Ensure Proper Spring Force

  1. Measure Spring Force in lbs.
  2. Calculate Spring Pressure
  3. If Any Spring Pressures Are Too Light, Replace All Springs

1. Measure Spring Force:

The first step is to get the right kind of scale depending on the type of spring you are measuring.

  • For constant force springs, hanging balance scales, like the Helwig DigiScale, are most popular.
  • For Helical coil spring, use a force gauge.

To Measure Constant Force Springs with the Helwig DigiScale

Before starting, it is important to know the approximate position of the coil for new and used brush.

  1. Turn ON the scale and ensure zero reading.
  2. Connect appropriate attachment to bottom loop of DigiScale, based on type of spring.
  3. Insert roller attachment inside constant force spring or under the red handle of Helwig wear indicator if applicable, slide strap around torsion spring finger, or hook turbine-style attachment under lip of Helwig wear indicator.
  4. Pull spring back to approximate new brush location (Figure A).
  5. SLOWLY let the spring recoil in the direction of brush travel.
  6. Observe spring force and take reading at approximately 1 outer diameter (OD) of the spring coil (Figure B).
  7. Repeat and record.
  8. Be sure to check ALL springs.

Figure showing starting and ending position of spring

A common mistake is pulling the spring to its maximum extension and measuring the force. Instead, pull the spring to the position it would be in with a new brush in the holder. Then, let the spring move downward in the direction of brush travel. Stop when only one coil’s outer diameter (OD) of the spring is left extended, and then measure the force. Measuring the force when the spring is fully extended or when it is under one coil’s OD can lead to significantly different results.

The finger style spring in the old ratchet style holder should be adjusted to position in use as values will change.

To Measure Helical Coil Spring:

These springs are not constant force, and their force constantly varies based on compression length throughout the life of the brush.

Instructions:

  1. Turn ON the force gauge and ensure zero reading.
  2. Connect appropriate attachment or stylus.
  3. Push or compress spring to approx. new brush position.
  4. SLOWLY release to free length.
  5. Repeat and record.

2. Determine If Spring Pressure is Within Range

Now that you have your spring force measurement, it is time to do a calculation to find whether the spring pressure is appropriate (too low is especially damaging but too high is also a problem).

Step 1: Complete the following equation

Spring Pressure Equation

Step 2 (only necessary if your brush has bevels on the top and/or bottom, i.e. if your brush is cut at an angle other than 90 degrees on top and/or bottom):

If your brush has bevels, you need to reduce the spring pressure found in Step 1 by the percentage listed in the “loss in downward force” column in the following graphic. If you have both top and bottom bevels, add the two percentages in the “loss in downward force” column together and reduce by the combined total percentage.

Loss of Spring Force - Beveled Brushes

Recommended Spring Pressure

Customers can always contact us to verify PSI calculations and provide comments and recommendations based on their application.

3. If you find that some spring pressures are too light, be sure to replace all springs!

Consistency of spring force is important for a long brush life.

To purchase the Helwig DigiScale to measure your constant force, torsion, and turbine style springs, click here.

For more information on the benefits of constant force springs, click here or contact us.