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Possible errors in accuracy due to couplings



1. Deviations in accuracy caused by torsion of a spring coupling (in particular shaft couplings).

This deviation in accuracy is defined by the torque to be transmitted (bearing friction and mass moment of inertia) and by the torsional spring constant of the torque stop.

The following applies: Max. error (degree) = max. torque [Ncm] / torsional spring constant [Ncm/Grad]

The following table serves to estimate the ratio between such an error and the smallest increment of an encoder:

Relationship between the resolution of an encoder in bit and the smallest increment in angular degrees:
Resolution binary
10 bit
11 bit
12 bit
13 bit
14 bit
17 bit
ppr
1024
2048
4096
8192
16384
131072
Increment degrees
0,352
0,176
0,088
0,044
0,022
0,0028
degrees:min:sec
0:21:06
0:10:33
0:05:16
0:02:38
0:01:19
0:00:10
sec
1266
633
316
158
79
10

2. Deviations in accuracy caused by radial play in the drive shaft with asymmetrical mounting of the couplings.

Here one has to differentiate between couplings that are mounted in an axially symmetrical manner round the shaft (all shaft couplings, many stator couplings) and asymmetrically mounted couplings (many stator couplings, all mounting brackets and pin-based torque stops).

With asymmetrical couplings deviations in accuracy can arise due to radial movements of the drive shaft (radial runout/play); this is determined by the system. These deviations are dependent on the amount of the radial play and the distance of the torque stop locating point from the drive shaft.
The relationship is shown in the following diagram:

Maximum permissible radial runout to achieve an accuracy >1/2 LSB when using an asymmetrical 1 point torque stop Genauigkeitsfehler_durch_Kupplungen

 

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