JQR 131.02 Snap Gage Training (Continued)

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131.02

Use and Care of the Snap Gage
(Continued)

Preparing to use the snap gage

Ensure that the snap gage, standard, and component to be measured are exposed to the same atmospheric conditions for 1 hour before any measurements are taken. Temperature variation between these items can result in erroneous readings.

Use the Bearing Shaft Diameter Limits table to determine the maximum and minimum limits of the bearing size installed. Thoroughly clean the surface to be measured. Select a plug standard that corresponds to the size of the bearing being used. Wipe the plug with a lint-free cloth. Set the plug standard on a clean, level surface. Wipe the anvils and backstop with a lint-free cloth.

Loosen both locking wheels. Use the size adjusting wheel to raise the upper anvil clear of the standard. Adjust the backstop to ensure that the standard centers between the upper and lower anvils.

With the standard on a level surface, bring the snap gage down until the lower anvil contacts the standard. Roll the gage forward until the upper anvil is centered on the standard. Use the size adjusting wheel to lower the upper anvil until the pointer rotates 1/2 to 3/4 of a revolution. Gently tighten both locking wheels.

Loosen the bezel clamp. Rotate the bezel until the pointer is over the zero. Tighten the bezel clamp and remove the standard. Use a rocking motion to place the standard back into the instrument. Check the zero.

Using the snap gage

Mark the shaft with a grease pencil in three positions, 120 mechanical degrees apart, around the circumference. Recheck the zero with a plug standard. Using a rocking motion, place the instrument on the shaft at position 1.

For bearing journals less than 1 inch, only six readings are required.

Measure the bearing journal at each marked position at three different places along the axis of the shaft for wide bearings and two places for narrow bearings. Record the readings and determine if they are within limits by referring to the Bearing Shaft Diameter Limits table. Remeasure the shaft if the readings are not within limits.

EXAMPLE: A 210 and a 310 standard bearing have an allowable range of 1.9690 to 1.9686. If the plug standard used to set the snap gage were 1.9687, then the zero mark on the dial would equal 1.9687. The allowable readings on the dial when the gage is placed on the shaft would be from +0.0003 to 0.0001.

Recheck the gage with a plug standard after each reading to ensure that the setting has not changed.

Do NOT hold the gage when it is not in use because the heat from the operator’s hand can affect readings.

Shafts with readings not within limits will require machining or replacement.

Clean the gage and standard by wiping with a lint-free cloth. Place the gage in its case. Stow the gage and standard.

Click on image to enlarge.

Snap Gage with Plug Standard for Calibrating

Review Questions
If you can correctly answer the questions below, you have learned the key points in this lesson. Choose the correct answer for each question, then look at your score below the questions. Review the lesson if you miss any questions.