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:
- Smoke around the bolt head or tension nut after installation.
- Sparking during installation.
- The bolt head or tension nut is hot after installation.
- Installed bolt torque substantially lower than bolter output.
- 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:
- Insert the bolt into the hole until the plate just contacts the roof.
- Tighten the bolt.
- 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
