Design Manual for Mercury Encoders: A Customer-Focused Guide

Celera Motion provides standard hubs for each standard grating. This article discusses the guidelines need to be followed to design a hub. After completing the hub design, customers are requested to send the hub drawing to the Applications Engineering group of Celera Motion for review. Once the hub has been fabricated, customers can send it to Celera Motion for mounting the grating to the hub. They can also mount the grating themselves. Nevertheless, the encoder system’s accuracy will be compromised if the mounting of the grating to the hub is improper.

The way of clamping the grating to the hub for the mounting and aligning process is illustrated in Figure 1.

The illustration above shows how the grating is clamped to the hub for the mounting and aligning process.

Figure 1. The illustration above shows how the grating is clamped to the hub for the mounting and aligning process.

Hub Design Criteria

Celera Motion suggests that the scale needs to be mounted with the grating reference surface (pattern side) that is in contact with the mounting surface of the hub. If the scale is mounted in this manner, then all grating thickness variations (can be up to +/-0.20mm) between the Z-axis alignment of the sensor head and the grating pattern can be eliminated. The recommended orientation of grating and hub in which the reference surface of the grating is in contact with the mounting surface of the hub is shown in Figure 2.

The illustration above shows the recommended orientation of grating and hub where the grating’s reference surface is contacting the hub’s mounting surface.

Figure 2. The illustration above shows the recommended orientation of grating and hub where the grating’s reference surface is contacting the hub’s mounting surface.

The less desirable grating orientation in which the grating’s back is in contact with the mounting surface of the hub is illustrated in Figure 3. This orientation will take into account the differences that can take place from one grating to the next into the Z-axis alignment tolerance and may demand the integration of axial hub adjustment or Z-axis sensor adjustment into the encoder mounting design.

The illustration above shows the less desirable grating orientation where the back of the grating is contacting hub’s mounting surface.

Figure 3. The illustration above shows the less desirable grating orientation where the back of the grating is contacting hub’s mounting surface.

Customers need to ensure that there is enough clearance between the I.D of the grating and the O.D. of the hub. Radial gap between hub shoulder and grating is 0.46 -0.00/+0.10mm (Figure 4). If it is unfeasible to mount the pattern side that faces the hub’s mounting surface, the grating pattern must not be placed directly in contact with the hub, because the sensor’s noise level can be increased by the laser light reflected off the hub (Figure 5).

Sufficient clearance between the O.D. of the hub and the I.D of the grating needs to be verified.

Figure 4. Sufficient clearance between the O.D. of the hub and the I.D of the grating needs to be verified.

Having relief undercut is recommended over having metal directly behind the structure.

Figure 5. Having relief undercut is recommended over having metal directly behind the structure.

It is recommended the shoulders on either side of the epoxy groove to have a minimum width of 2mm. The epoxy groove’s depth and width need to be verified. The shoulder of the hub must have a width of at least 3.1mm to provide adequate clearance for mounting screws.

The bottom surface of the epoxy groove must have a rough finish as a very smooth surface will affect the adherence of the epoxy and consequently the bonding between the hub and the grating. It is necessary to verify that the size of the three threaded mounting holes as well as the number of epoxy holes is correct (Figure 6). The specifications are listed in the following table.

It is necessary to verify that the size of the three threaded mounting holes as well as number of epoxy holes is correct.

Figure 6. It is necessary to verify that the size of the three threaded mounting holes as well as number of epoxy holes is correct.

Epoxy Groove Diameter Number of Epoxy Holes Epoxy Hole Size Number of Taped Holes Thread Size
Up to 0.42 [Up to 10.7] None None None None
0.42 to 0.7 [10.7 to 17.8] 3 .062 [1.55] 3 0-80 UNC [M1.7]
0.7 to 1. 2 [ 17.8 to 30.5] 3 .062 [1.55] 3 2-56 UNC [M2]
1.2 to 2.2 [ 30.5 to 55.9] 3 .062 [1.55] 3 8-32 UNC [M4.2]
Above 2.2 [55.9] 6 .062 [1.55] 3 8-32 UNC [M4.2]

inch [mm]

The recommended hub surface finish is Ra = 0.8mm. Celera Motion suggests that hubs are to be fabricated out of 303/304 stainless steel with clear passivate coating. After breaking all corners and removing all burrs, the perpendicularity between the mounting surface and the hub bore is verified for tolerance of 0.001". Then the parallelism between the rotary glass scale surface and the hub mounting surface is verified.

Hub Design Check List

The grating reference surface (pattern side) is in contact with the hub’s mounting surface (select one).

  • Yes (recommended)
  • No (not recommended)
  • The clearance between grating inside diameter and the hub’s shoulder is adequate.
  • The epoxy groove depth and width, landing on either side of the groove, and surface finish of the groove bottom are all correct.
  • The size of the three threaded holes for the mounting clamp is correct.
  • The number and size of the epoxy holes are correct.
  • Indicate reference diameter on hub drawing (generally the hub bore)
  • Indicate reference surface on hub drawing (generally same side as epoxy groove)
  • All corners broken and burrs removed from the hub.
  • The perpendicularity between hub bore and grating mounting surface is correct.
  • The parallelism between grating mounting surface and hub is correct.
  • Verify material type, surface finish, and coatings are all correct.
  • A Celera Motion Applications Engineer has reviewed the hub design.

Celera Motion

This information has been sourced, reviewed and adapted from materials provided by Celera Motion.

For more information on this source, please visit Celera Motion.

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