Is it important for a shaft key to be ground flat?

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Discussion Overview

The discussion revolves around the importance of grinding shaft keys flat, particularly focusing on the implications of surface waviness on performance and fit within mechanical assemblies. Participants explore the tolerances of key dimensions, the effects of surface irregularities, and the role of keys in securing components on shafts.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes their company's practice of grinding shaft keys on four surfaces and questions the impact of surface waviness on shear area and overall performance.
  • Another participant discusses the fit between the key and slot, suggesting that if the high points are within specification, the waviness may not pose a problem, but raises questions about measurement accuracy.
  • A third participant emphasizes that only two sides of the key should be critical, questioning whether the waviness is present on the measured sides.
  • A fourth participant notes that keys are meant to fix the phase angle of a hub and should not be subjected to dynamic loads, suggesting that a spline is more appropriate for such situations.

Areas of Agreement / Disagreement

Participants express differing views on the significance of surface waviness, with some suggesting it may not impact performance while others raise concerns about the implications for fit and load distribution. The discussion remains unresolved regarding the necessity of flatness in shaft keys.

Contextual Notes

There are uncertainties regarding the measurement techniques used and the definitions of acceptable tolerances. The discussion does not clarify the specific conditions under which the keys will be used or the potential effects of dynamic loads on key performance.

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How important is it for a parallel shaft key to be ground flat? If key measures the right size but the surface isn't especially flat, will there be any negative affects?
Our company has made our own shaft keys for many years, grinding them on the 4 loaded surfaces.
For example, let's say the pulley and shaft key seats are .5005 +/- .0005 wide.
We've made ground keys that are .4995+/- .0002 with a flatness of .0004.
We now are considering purchasing keys with a tolerance of .4990+/- .0005
The keys are measuring this width in tolerance, as long as you use a calipers or a micrometer with reasonably large anvil. However, if you put a dial indicator across it, with the key on a surface plate, The needle moves about .0015". So there is waviness to the key surface, but the high points measure in spec.

My thought is that this variation has minimal affect on the shear area, so there should be little to no difference in strength or performance. However, the key will not be bearing load onto the key seat as uniformly as if they were ground keys.

Can anyone see any issue with these slightly wavey surface keys?

Pictures showing examples of key seat in a shaft, in a gear, and in an assembly.
1737153343817.png
 
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If the slot width is 0.5000...0.5010, and the key width is 0.4993...0.4997, resulting in a gap fit. If we add 0.0015 to the width of 0.4990, we get 0.5005, that is, a normal fit or with a weak tension. If the highs are within the specification, it is unclear why the readings are 0.0015. Maybe +0.0005 on both sides and the error of the device? Personally, it seems to me that there is no problem. But in general, for such mating, as far as I know, a low-tension landing is used in order to eliminate backlash and impacts during launch
 
Only two sides of the key should matter.
The attached drawing shows a key which top surface must have a gap.
Are the four sides showing the measured waviness?
 
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A key is used to fix the phase angle of a hub on a shaft. Initially, the key will contact the keyway wherever, both will deform, until a static situation is reached. Some form of clamp or taper must be provided, to lock the hub securely on the shaft.

A key should not be in a dynamic situation, that is the job of a spline. Where the sides of the key is carrying a dynamic load, it will flog out, damaging the key and the keyway.
 
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