No. 532 September 2008 

Collapsible Drill Steel Enclosure for Reducing Roof Bolting 

Machine Drilling Noise
 

Objective 

To reduce operator exposure to drilling noise from roof 

bolting machines. 

Background 

Noise-induced hearing loss (NIHL) is a prevalent occupational illness in the United States. Studies indicate that 70%–90% 

of miners have NIHL significant enough to be classified as a 

hearing disability. Mine Safety and Health Administration 

studies indicated that roof bolting machine operators are among 

those overexposed to noise. Field studies support the premise 

that, on average, drilling is the loudest noise source to which a 

roof bolting machine operator is typically exposed and contributes greatly to the operator’s noise exposure. The National Institute for Occupational Safety and Health (NIOSH) has further 

determined that the drill steel and chuck radiate a significant 

amount of noise during drilling. To address this noise hazard, 

NIOSH developed a collapsible drill steel enclosure (CDSE) to 

encapsulate the drill steel during the drilling portion of the roof 

bolting machine duty cycle. In addition, NIOSH is collaborating 

with J. H. Fletcher & Co. in the continued development of a 

CDSE. 

Technical Approach 

A proven method to reduce noise transmission from a source 

(drill steel/chuck) to a receiver (operator) is to create a barrier 

around the source. With this in mind, NIOSH developed a CDSE 

to break the noise path between the drill steel and the operator. 

The CDSE is composed of a bellows (barrier between the source 

and operator), spring (to support the CDSE), hinge (to facilitate 

moving the CDSE for roof bolt installation), cap (acoustic barrier 

at the top of the CDSE), and mounting hardware (for installation 

on the roof bolting machine). 

The device is simple to use. Before drilling, the operator 

installs the drill steel through the CDSE and into the chuck. At 

the beginning of the drilling cycle, the CDSE is fully extended. A 

gap between the cap and the roof allows the operator to see the 

drill bit/media interface and start the hole (Figure 1). As the 

drill chuck rises, the gap closes until the cap is flush with the roof 

(Figure 2). The CDSE then collapses as drilling continues 

(Figure 3). When drilling is completed, the operator swings the 

CDSE to the side and installs the roof bolt as usual (Figure 4). 

Figure 1.—The CDSE at the start of drilling. The gap above the 

CDSE allows the operator to see the drill bit/media interface to 

start the hole. The height of the CDSE can be varied to suit the 

particular application. 

Results 

Laboratory testing in a reverberation chamber (for sound 

power level) and hemianechoic chamber (for noise source 

identification and operator ear sound pressure level) showed that 
the CDSE reduces the noise level at the roof bolting machine 

operator location, thus reducing noise exposure. This was 

confirmed by field testing at a cooperating mine. NIOSH 

conducted time-motion studies to determine the effect of using a 

CDSE on operator noise exposure and drilling noise. Field data 

shown in Figure 5 confirm that using the CDSE reduces an 

operator’s 8-hour time-weighted exposure from 97 to 91 dB(A), 

a 6-dB(A) reduction. NIOSH conducted additional analysis of 

only drilling noise. Using a CDSE reduced the operator drilling 

noise levels from 107 to 100 dB(A), a 7-dB(A) decrease. 

Figure 2.—The CDSE is now flush with roof. 

Figure 3.—As drilling advances, the CDSE collapses. 

Figure 4.—Once drilling is completed, the CDSE is moved aside 

so the roof bolt can be installed. 

 ent noPerc ise dose 

Time 

Figure 5.—Field data show that using a CDSE reduced a roof 

bolting machine operator’s 8-hour time-weighted exposure from 97 

to 91 dB(A). 

For More Information 

For more information on the CDSE for reducing noise 

exposure, contact J. Shawn Peterson (412-386-4995, 

JPeterson@cdc.gov) or Lynn A. Alcorn (412-386-6732,
 LAlcorn@cdc.gov), NIOSH Pittsburgh Research Laboratory, 

P.O. Box 18070, Pittsburgh, PA 15236-0070. 

To receive NIOSH documents or for more information about 

occupational safety and health topics, contact: 1–800–CDC– 

INFO (1–800–232–4636), 1–888–232–6348 (TTY), e-mail:

cdcinfo@cdc.gov, or visit the NIOSH Web site at http:// 

www.cdc.gov/niosh 

Mention of any company name or product does not constitute 

endorsement by the National Institute for Occupational Safety 

and Health. 

DHHS (NIOSH) Publication No. 2008–146