Throughout my career, people have asked which type of measuring wheel is the best for use with a rotary encoder. The short answer is, whichever one gives you the best result on your specific application. Obviously, the real answer is a bit more complicated.
There are four main types of encoder wheel:
- Rubber or Rubber-coated
- Knurled Steel or Aluminum
It is my opinion that rubber, or rubber-coated measuring wheels should never be used. Rubber expands and contracts with temperature changes.
It becomes more pliable with heat and more rigid with cold. This changes the physical diameter of the wheel, and you can never be sure of exactly how much compression you’re getting at any given point in the wheel.
I’ve never seen a magnetic wheel that offered an advantage over a phenolic or knurled steel wheel of the same diameter. Magnetic wheels will pick up bits of metal debris, becoming a irregularly shaped wheel, at best. Operators are notoriously unreliable in keeping the wheel clean and free of build-up.
Measuring wheels should suit the application. Painted materials like those used for products that will be visible to the consumer in their finished form usually require a measuring wheel that will not mark the material. Metal wheels are generally not acceptable for such processes. There are several types of polymer wheels that are slick enough to avoid scratching the formed part, yet offer enough friction to track well. In these situations, an encoder should be chosen that offers a very low-friction bearing assembly, so there is less friction in the bearing than between the wheel and the material. Encoder shaft bearings that produce too much drag will cause the wheel to slip.
Phenolic wheels are appropriate for painted steel, or for stainless when the shiny quality of the stainless is for aesthetic purposes. Otherwise, if you can get away with leaving small marks, a knurled steel wheel is best on stainless.
Hardened steel makes an excellent measuring wheel. It can be knurled so that it “bites” the material. This type of wheel will generally mark the material, but if this is aesthetically acceptable for the finished product, less tension is required to track the material. The marks left on material by a knurled wheel can be beneficial as a quick check for tracking quality. Often, a finished part can be held under the glare of shop lights to make the “tic” marks left on the steel visible to the naked eye. The quality of the encoder tracking can be easily checked based on the type of tracking patterns left on the steel.
Regardless of the encoder wheel material, it is critical that the wheel be as concentric (perfectly round) as possible. The best wheels are round to ±0.00025″ or better. The more critical the tolerance and repeatability requirement, the more important the ’roundness’ or concentricity becomes. Even slightly out-of-round wheels can generate inconsistent results, usually in the form of long-short pieces in cases where part lengths are exact multiples of the measuring wheel circumference.
For best results, encoder shafts should always be directly coupled to the measuring wheel, and the wheel should always ride directly on the material. Often, maintenance or engineering will construct a bracket that uses a flex (“zero backlash”), dual-spring, or spline couplers to isolate the encoder from the measuring wheel. This is usually done to protect the encoder from material crashes. Encoders are relatively cheap, and if the material has a tendency to crash in a certain area of the machine, this should be a flag that further engineering (or maintenance) of that machine element is required.
All too often, these couplers create long delays in troubleshooting problems with the encoder system. Flex couplers can break and allow backlash, but not so much that the damaged portion of the coupler is easy to see. The inner spring of a dual-spring coupler will break allowing significant backlash, but the problem can’t be seen until the coupler is physically removed. Spline couplers are notorious for inducing backlash over time, as the spider insert begins to wear.
Machine builders will sometimes couple the encoder shaft to a roll tool or pinch roll. This always results in poor encoder tracking. Unless the rolls are designed to ensure perfect tracking between the roll and material movement (i.e. embossing wheels), it is far better to use a separate, low-mass measuring wheel.