Various designs of expansion units are available and are usually referred to as either the standard type or the bail type.

Characteristics of Types

Slotted Unit

1 Obtains hold in rock by cutting its way into the wall of the drilled hole and by replacing the cut rock with the steel of the wedge. If the rock is difficult to cut with the soft steel, it compresses and deforms the steel

2. When properly installed under favorable rock conditions and it has a very wide range of application it will bear its load efficiently well past its yield point,

3. It is easily manufactured.

4:

Installation loading is in tension, and the torque applied to tighten the nut does not extend through the length of the bolt, and is therefore not weakened through normal installation.

5. The driving to anchorage of the wedge in the slot increases the installation time over the expansion-shell type about 20 percent.

6, The overall cost for material is usually greater.

7. It loses its efficiency in the softer, semi-plastic rocks, such as shales.

8. It leaves a protruding bolt end, extending downward from the roof, creating a hazard

Expansion-Shell Unit

1. Works by compressing the wall of the drill hole by expanding the shell, thus performing best in softer, more plastic rock. If material is too soft, bolt tension cannot be maintained because of slippage of the expanding shell.

2. The weakest area of the unit is the threaded portion of the female nut and the male threads of the bolt rod

3. The torque required to activate the device is supplied at the bolt head and any "galling" at either the bearing plate or the expansion unit increases the torque requirements for proper fastening and decreases the overall efficiency of the bolt.

4. The cost ordinarily is about 25 percent less than that of the slotted-type bolt.

5. Can be installed quickly and uniformly.

6.

Equipment for installation can be powered either by electricity or compressed air.

Suspension vs. Conventional Timbering Effectiveness

Many hundreds of tests have been conducted on roof bolts by various elements of the mining industry since their adoption in 1947, principally to determine the effectiveness of suspension support as compared to conventional timbering.

Figure 18 is a graph showing the differential roof sag that developed over a period of 160 days in entry roof supported by conventional timbering and by suspension timbering.

Specifications for Roof-Bolting Material

In an attempt to develop a measure of consistency in the quality of materials used by manufacturers in fabricating roof-bolt assemblies and for the establishment of minimum dimensions, a committee, representing all interested groups, developed certain guide lines for the several sizes of bolts used by the industry. Following are the specifications for a 5/8-inch roof bolt, so chosen for our example because of its wide acceptance for universal use by the industry. (Similar specifications have also been established for 3/4-inch and 1-inch bolts.)

AMERICAN STANDARD SPECIFICATIONS

for

ROOF-BOLTING MATERIALS

IN COAL MINES

Sponsored By

American Mining Congress

Approved By

American Standards Association

August 1957

Purpose of
Recommendations

10 20 30 40 50 60 70 120 130 140 150 160 170

Figure 18.

Elapsed Time After Installation (Days)

Differential roof sag. (Conventional timbering vs. roof bolts.)

1. Safety. To protect the consumer by assuring that roofbolting materials shall be made in accordance with certain specifications pertaining to quality and minimum dimensions. It must be remembered that the lives of mine workers are dependent upon an effective roof structure provided by roof bolting. In all safe structural designs, it is common practice to avoid loading materials beyond their elastic limits. This should be considered an inherent practice in roof bolting. Over-loading during installation should not be allowed. Therefore, these specifications, which require minimum tensile properties for the bars from which roof bolts are made, are very necessary. The minimum yield loads shown on the accompanying drawings for each bolt are particularly significant as far as practical applications are concerned.

2. Interchangeability. To establish certain manufacturing practices to provide a uniformity of product without suppressing improvements in design and advancements in engineering.

Specifications for Roof Bolts

Steel Specification. All mine roof bolts shall be made of openhearth or electric furnace steels according to Tentative Specification for Carbon-Steel Bars subject to Mechanical Property Requirements ASTM Designation: A-306, latest issue.

Bars from which roof bolts are made shall have tensile properties shown in Table 7.

Bolt Dimensions. In Figure 19 is shown essential, basic, and minimum dimensions of 5/8-inch square-head roof bolts. They are not intended to preclude the use of special reinforced, recessed, or self-centering heads if the consumer so desires.

Thread Fits. All threads shall accept a Class 2 "go" ring gage.

Bolt-Head Markings:

1.

The length in inches shall be marked on each bolt head. For example, the figure 60 shall indicate a 5-foot bolt.

2.

All 5/8-inch high strength bolts shall be marked with a five-point star of approximately 1/4-inch in size.

turer.

3. Each bolt head shall be marked to identify the manufac