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District 8 - Coal Mine Safety and Health
Info-Grams and Close Call Accidents

Close Call Graphic
Department of Labor
Mine Safety and Health Administration



Jam Nut & ShellA mine in District 8 recently experienced several roof falls on a working section panel. Several roof bolts were recovered from one of the falls and inspected. The bolts were expansion shell/resin assisted type bolts using a jam nut. The bolts recovered had severely twisted shells with the leaves buckled inward onto the threads. Although some geologic conditions had an impact on the deteriorated roof conditions in the area, the distorted shells on the bolts may have significantly contributed to the roof falls.
    The roof bolts used a standard expansion shell with a jam nut and were installed with 2 feet of resin. Tests have shown that when the jam nut is too strong, the nut will not strip off and the expansion shell will then twist and deform during tensioning of the bolt. The results are low tension/torque ratio, shell distortion, high thread tension, or some combination of these. When this occurs the bolt is not functioning properly. For expansion shell/resin assisted bolts, the jam nut should have a breaking torque less than or equal to the installed torque of the bolt.

  • Make sure products received from manufacturers are certified as meeting all applicable ASTM specifications as stated in 30 CFR § 75.204. Jam nuts should not interfere with the proper tensioning of the bolt.
  • Check roof bolting machines for proper torque settings. Abnormally high torque may also add to the problem of distorted expansion shells.
  • Re-instruct roof bolting machine operators as to proper installation procedures, such as proper drill hole depth, resin cartridge length, and installed torque. Follow the manufacturer's installation procedures.

If you would like further information on this subject, please contact the MSHA District Office at 812-882-7617.
    Please post this alert notice in a conspicuous location at the mine.

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This information was provided by concerned miners in an effort to eliminate accidents. Accident or "close call" information within District 8 may be shared by contacting the MSHA office at (812) 882-7617.