MSHA - Directorate of Technical Support A Troubleshooting Guide for
Roof Support Systems


Appendix A

A.1 EXCESSIVE THRUST

Excessive thrust affects the installation by introducing additional friction which effectively increases the torque requirements for proper installation.

Some of the indications of excessive thrust are:

  1. Smoke around the bolt head or tension nut after installation.
  2. Sparking during installation.
  3. The bolt head or tension nut is hot after installation.
  4. Installed bolt torque substantially lower than bolter output.
  5. Bolt head and/or washer welded to plate, or severe galling.

A simple check to determine if excessive thrust is causing the problem is to have the operator install several bolts in the following procedure:

  1. Insert the bolt into the hole until the plate just contacts the roof.
  2. Tighten the bolt.
  3. Check the installation for the indications listed above. if none are evident, then excessive thrust should be ruled out.

A.2 SEVERE INSTALLATION ANGLE

A severe installation angle will place eccentric loads on the roof bolt components which can result in premature failure. If you suspect severe installation angle is causing the problem install several bolts in an area where the roof is relatively flat and see if the problem has been eliminated.

A.3 APPLIED TORQUE TOO HIGH/LOW

Torque variations can cause supports to be installed improperly which could result in component failure. Check the installed torque of several bolts immediately after installation. If the torque is too high or low check the bolter with an in-line torque meter or similar device.

A.4 PLATE/BOLT MISMATCH

For an effective support it is important that the strength of the plates matches the strength of the bolts they are used with. Check the grade rating of the plate to determine if it is capable of supporting the ultimate load of the bolt or the installed load. EXAMPLE: A grade 2 plate has a minimum ultimate load of 20,000 lbs. when tested over a 4" diameter hole as described in ASTM F432- 88. The minimum ASTM ultimate load of a 3/4" dia. grade 75 bolt is 33,400 lbs. In this example you can expect the plate to fail before the bolt breaks.

A.5 BAIL/SUPPORT NUT TOO STRONG

If the bail or support nut is too strong the result can be low tension/torque ratio and malfunctioning or broken anchorage units. To determine if the bail/support nut is too strong, place the bolt in a borehole with the head about 4" below the roof line. Set the anchor by turning with a torque wrench. If the anchorage unit turns in the hole remove the bolt and pre-expand the anchor, re-insert the bolt and tighten. Continue turning the bolt with the torque wrench until the support nut or bail breaks free. For normal installations without resin the breaking torque should be low (30-50 ft.-lbs.). When bolts are to be used with resin the breaking torque will be higher but should not exceed the installed torque.

A.6 DEFECTIVE ANCHOR

When checking for defective anchors, visually inspect several anchors looking for cracks in the leaves or plugs. Also check the alignment of the plug in the shell and for any protrusions on the plug and shell that might prevent the anchor from setting. It is also possible that the plug threads are seizing on the bolt due to lateral compression. If you suspect plug thread seizure, contact the manufacturer.

A.7 BAIL/SUPPORT NUT TOO WEAK

If the bail or support nut is too weak the shell can fall off the plug either during insertion or before it gets a sufficient bite along the borehole wall. To check the bail/support nut turn the shell by hand. The bail/support nut should be strong enough to allow the plug to expand and firmly set the shell without breaking.

A.8 WASHER TOO HARD OR TOO SOFT

Washers that are not properly hardened can result in a tension/torque ratio much higher or lower than expected. As a field check install some bolts outby then take them out and visually inspect the washers. If there are signs of cracking or excessive galling then check the washer hardness on Rockwell hardness tester. The washers should be 35-45 on the Rockwell C scale.

A.9 DEFECTIVE THREADS

A thread clearance that is too tight can result in low tension/torque ratio or complete lock-up of the support. A check for this condition would be to take the fastener e.g., plug, tension nut or coupler and thread it onto the bolts (remove any shear mechanisms from the fastener before performing this check). The fastener should thread easily down the entire length of the bolt threads. If the fastener locks up or excessive resistance is felt, visually inspect the bolt threads for flat threads or burrs. Check several bolts with several fasteners to determine if the problem is with the bolt or fastener threads. A thread clearance that is too loose is very serious since thread stripping can occur resulting in premature support failure. This condition can only be checked with thread gages.

A.10 DEFECTIVE BOLT

If sufficient anchorage is available, conduct a pull test to determine the ultimate strength of the bolt. When checking bolts underground it is not necessary, nor recommended to break the bolts. Loading the bolts to the minimum ultimate load is sufficient since most bolts have breaking loads in excess of the ASTM minimum. Since this test is to determine if the bolts are defective, precautions should be taken to prevent injury if the bolt should break prematurely. If a pull test cannot be conducted the bolts must be sent to a testing lab for analysis.

A.11 DEFECTIVE PLATE

Defective plates can result in the complete loss of support. When a plate failure is observed, first determine if the proper grade plate is being used (see A.4 PLATE-BOLT MISMATCH ). If the bolt and plate match up well and you still suspect the plates of being defective send the plates to a testing lab for analysis. If the tension/torque ratio is erratic, check the edge of the hole for burrs.

A.12 PLATE HOLE TOO LARGE

The center hole of bearing plates cannot exceed 1-1/2 inch. If the 5/8" bolts have small heads the plate hole should be sized accordingly to prevent pull through.

A.13 LIMITED THREAD ENGAGEMENT

Limited thread engagement can result in premature bolt failure due to thread stripping. The bolt threads should be at least 3-3/4 inches long. The plug should have a tapped length equal to one times the nominal bolt diameter with which they are to be used. Also observe the operator during assembly of the bolts and check to see if the bolts are fully engaged in the coupler.

A.14 HOLE TOO SHORT

Measure the length of the hole. The hole should be drilled at least 1-inch longer than the length of the bolt to allow for thread take-up.

A.15 HOLE DIAMETER TOO LARGE/SMALL

Hole diameter can vary in hard or soft strata. Check hole diameter at the anchorage horizon. If the hole diameter is not within the anchor manufacturer's recommendation check the bits which should be plus or minus 0.030-inch of the manufacturer's recommended hole size.

A.16 PLATE-BOLT GALLING

Galling introduces additional friction which effectively increases the torque requirement for a properly installed support. To check for galling have the bolter install some bolts, then remove them so that a visual inspection of the plate and bolt head flange can be made. If galling is evident only in some instances this could indicate poor installation procedure e.g., excessive thrust, applied torque too high, etc. If galling occurred for each installation check for burrs around the plate hole, or excessive flash on the bolt flange. Galling can also be caused by a plate/washer mismatch (see A.17 below). The use of a hardened washer can reduce galling during installation.

A.17 PLATE/WASHER MISMATCH

A poorly designed plate embossment or a large washer can cause galling if either condition prevents the bolt head from seating completely on the plate. If the washer cannot seat flat on the plate, then during installation the washer will bend. When this occurs the bolt head and washer are not flat against each other and the edges of the bolt flange gall the washer.

A.18 ANCHORAGE EXCEEDED

To determine the maximum anchorage available a pull test must be conducted. If it is determined that the anchorage available is not sufficient for the particular application, then further action must be taken. The cause could be either poor roof strata, or the anchor design is not suited to the strata at that horizon. The easiest check of the anchor design is to move the anchorage horizon, then conduct more pull tests. If this does not remedy the situation the next step would be to try a different anchor design.

A.19 POOR ROOF STRATA

The check for poor roof strata would be the same as for ANCHORAGE EXCEEDED (A.9). If it is determined that the problem is due to poor strata the only remedy might be changing to a different roof support system e.g., fully grouted bolt or resin anchored tensioned bolt.

A.20 BOLT/ANCHOR THREADS JAM

To determine if the anchor threads have jammed perform the check as explained in DEFECTIVE THREADS. If the threads check out all right, the problem may be that the plug threads are seizing on the bolt due to lateral compression. If you suspect plug thread seizure, contact the manufacturer.

A.21 THREADS GALLED ON INSTALLATION

To check if the threads are galling on installation have the bolter install several bolts outby. Then loosen the bolts with a torque wrench. If the torque needed to turn the bolt is high after the tension is off the bolt, or if the bolt turns but cannot be removed then the threads have galled. Check the threads of the bolts that could be removed for worn or shiny areas. If galling is suspected perform the checks explained in DEFECTIVE THREADS (A.9).

A.22 ANCHOR PRE-EXPANDED

A pre-expanded anchor will be difficult to insert in the hole by hand, or require excessive thrust to push the bolt into the hole. If it is difficult to insert the anchor, check the position of the plug in relation to the shell. If the plug is engaged too far into the shell prior to installation the anchor will drag along the borehole wall and could break during insertion. Re-position the plug and the leaves of the shell and insert the bolt. The drag on insertion should be reduced, if not re-position the plug again.

A.23 IRREGULAR ROOF SURFACE

See A.2 SEVERE INSTALLATION ANGLE