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Backlash;
When mechanical parts are manufactured, the manufacturing process results, and in some cases actually requires, that these parts are built with some "slop" in their finished dimensions.
"Slop!" How’s that for a technical term?
The "slop" is there because the machine that's being used to make a specific part may be out of specification, is being run too fast, too slowly, or too hard, the operator isn't paying attention; it could be a host of reasons, and the result is that the parts that are being built aren't consistent in size.
Since oversized parts almost never work, the tendency would then be to err on the side of caution (and to reduce waste & cost) and try to keep the parts slightly undersized so that if there are variations in the finished size of the part, given that they are all undersized, they will still likely work in the end assembly or application for which they were manufactured.
Also, "slop" sometimes has to be designed into parts. They have to be machined or cast or molded to be a bit undersized for them to actually be able to be assembled into the finished assembly. There has to be room for gear-to-gear tooth movement for example, or space must be allowed for lubrication to flow between moving parts.
When all of the slightly undersized parts are assembled into the finished item, and that finished item is put into operation, for example, a pneumatic rotary actuator , backlash will occur during every cycle and affect it's operation.
As each part in the rotary actuator begins moving by being impacted by the previous part that’s a little under-sized, then from the time the “go signal” is given for that actuator to work, there may be an appreciable amount of time before the tooling actually moves.
Positions of the end of arm tooling will not be exact, end of arm tooling movement will result, and there will be a lag time from when the actuator piston starts to move with the ingress of compressed air, to the time the tooling starts to move.
When air is supplied to one of the air ports on the vanes or the piston inside the rotary actuator, these parts will start to move. Depending on the accuracy of the machining of the parts (and this usually correlates to the cost of the rotary actuator) the vanes and piston may move quite a bit before their motion affects the shaft. This "play" is backlash.
By the same token, when the rotary actuator has reached end of stroke, the vanes or the piston will stop before the shaft does.
Or, if an external stop is used for the tooling, when the tooling is stopped it will stop the shaft, but the vanes or piston inside the unit may continue to move a bit.
Depending on the complexity of the finished device, the more parts are joined together, the greater the "tolerance stack-up" and the greater the potential for backlash in the finished assembly.
If every part in that piece of equipment is slightly undersized, by the time you add up all of the disparities in the multitude of parts that is assembled into the final whole, there could be quite a bit of backlash to be engineered out of the final usage of that piece of equipment.
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