Improved NIOSH Collapsible Drill Steel Enclosure (CDSE) Prototype Helps Reduce Roof Bolting Drilling Noise

The National Institute for Occupational Safety and Health (NIOSH) is seeking manufacturers to develop its improved prototype of a collapsible drill steel enclosure (CDSE) into a product for use by mines and other industries. This NIOSH-developed engineering control reduces emissions of hazardous noise during the operation of roof bolting machines in underground coal mines. Based on stakeholder feedback, NIOSH has redesigned its CDSE for roof bolting machines from the original prototype to further improve its usability and durability in the field. This noise control technology has been proven to reduce noise emissions when drilling holes during roof bolting. The new, improved prototype addresses previous end-user concerns about the suitability of the device in an underground coal mining environment, and could be applied in other industries where noise exposure from drilling is a concern.


Exposure to hazardous noise in underground coal mining can lead to noise-induced hearing loss (NIHL), a common occupational illness in the United States. Exposure data for underground coal mining from 2002 to 2011, collected by the Mine Safety and Health Administration (MSHA), indicates that roof bolting machine operators are susceptible to hazardous noise overexposure [Azman and Sun 2017]. NIOSH field studies support the premise that drilling creates the highest noise levels that roof bolting machine operators are exposed to, and therefore is the greatest contributor to the operator’s time-weighted average noise exposure. Previous studies have also identified the roof bolter’s drill steel and chuck as primary sources for noise radiation when drilling [NIOSH 2008].

In response to these findings, NIOSH developed a collapsible drill steel enclosure (CDSE) prototype to enclose the noise radiating from the drill steel. The CDSE proved to be effective in reducing the noise produced during the drilling of roof bolt holes in both laboratory and field studies. Through iterative design and testing of the original CDSE prototype, NIOSH researchers have successfully developed a refined version with improved usability, resulting in a new CDSE prototype.

Original Prototype CDSE Design

Introducing a barrier between the noise source and the operator is a primary method of reducing noise exposure. The original CDSE prototype successfully employed this method to reduce operator noise overexposure, but the design proved to be too cumbersome to practically use in the field. Further, there were safety concerns about not being able to see any bowing of the drill steel due to the CDSE—too much bow can cause the drill steel to snap. Despite these difficulties with implementation, NIOSH successfully met the goal of noise reduction, which led researchers to seek to improve the CDSE design while maintaining its ability to reduce operator noise exposure.

New Prototype CDSE Design

Photograph of researcher performing laboratory testing of the new collapsible drill still enclosure prototype.

Figure 1. Laboratory testing of the new collapsible
drill still enclosure prototype. Photo by NIOSH.

The new CDSE design (see Figure 1) is a 1.75-inch-diameter bellows which attaches to a top coupler with an aluminum housing and a sealed roller bearing. This allows the bellows portion of the CDSE to rotate with the drill steel independently from the top bearing, which maintains contact with the mine roof. The water-resistant bellows is manufactured from materials consisting of neoprene, buna, butyl, viton, silicone, and Goralon.

The redesigned CDSE has an aluminum bottom coupler holding the bottom of the bellows to the drill steel. Inside the device’s top and bottom couplers, spring-loaded ball bearings hold the end pieces to the drill steel with friction while allowing the CDSE to slide up and down over the drill steel. Importantly, the improved device is no longer attached to the roof bolting machine, and instead is affixed to the drill steel with attachments at each end. Click here to download a PDF of detailed schematic drawings of the collapsible drill steel enclosure.

The CDSE is fully extended at the start of drilling, and collapses as the boom of the roof bolting machine rises when drilling into the mine roof. This allows the device to remain in use for the duration of each drilled bolt hole. The slimmer design of the new CDSE prototype also enables the operator to observe any bowing of the drill steel, allowing for greater visual feedback of drilling performance. Finally, the CDSE can also remain attached to a drill steel when not in use, as the device is no longer attached to the machine itself.

Benefits of the Redesigned Technology

One of the major benefits of the new CDSE technology is its ease of use. Prior to drilling, the operator slides the CDSE onto the drill steel and attaches it using the spring-loaded balls. The device remains on the drill steel throughout the drilling shift and does not require removal prior to the installation of roof bolts. In addition, because the device can remain on the drill steel while not in use, a separate storage area is not required. The CDSE adds no additional length and is only minimally wider than a standard drill steel, enabling it to be easily stowed away when not in use due to its flexible tubular design.

The new CDSE prototype addresses the three main deficiencies of the original prototype model: It is smaller and takes up less space when not deployed, the material is less likely to become damaged from normal use, and it allows the operator to see any bowing of the steel while drilling. Overall, this redesigned CDSE shows significantly improved usability compared to the previous prototype.

Findings and Limitations

Laboratory testing of the new CDSE prototype on a roof bolting machine in NIOSH’s hemi-anechoic chamber showed a 4–5-dB(A) reduction of the noise level at the operator’s location [Azman et al. 2014]. Unpublished results from field testing have shown the device to be durable and user-friendly under typical mining conditions, with similar noise reduction capability to that found in the laboratory—3–5 dB(A) dependent on drilling conditions at the specific field site.

Based on these findings, the improved CDSE prototype has the potential to reduce the eight-hour time-weighted average noise exposure of the roof bolting machine drill operator. However, it can only reduce the noise radiated during the drilling operation, and will not affect the noise radiated during tramming or bolting. Further, as compared to results obtained during laboratory evaluation, the extreme variability in mine roof conditions may produce a large variance in noise reduction during field evaluation and operational use of the device.

Finally, a manufacturer would want to strike the right balance between material selection and acoustic benefits, and to determine whether any bowing of the drill steel is clearly visible while using the device to ensure safety. The closer the bellows rides to the drill steel, the easier the bowing is to observe.

Availability for Commercialization

This technology was developed and tested for application on roof bolting machines in underground coal mining. However, a collapsible shroud noise enclosure concept could also apply to many other drilling operations where drilling noise poses a problem for workers, such as those found in other mining commodities, construction, oil and gas extraction, and certain road and marine installation activities. NIOSH is seeking potential manufacturers for further refinement of the new CDSE prototype and commercialization of this technology.


The findings and conclusions about the CDSE prototype are those of the authors and do not necessarily represent the official position of the National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention.


Azman AS, Sun K [2017]. Surveillance data and noise control development in mining, Proceedings of NOISE-CON 2017, Grand Rapids, MI. Institute of Noise Control Engineering.

Azman A, Camargo H, Alcorn L [2014]. Laboratory evaluations of a redesigned collapsible drill steel enclosure to reduce noise from roof bolting machines. Proceedings of NOISE-CON 2014, Ft. Lauderdale, FL. Institute of Noise Control Engineering.

NIOSH [2008]. Technology News 532—collapsible drill steel enclosure for reducing roof bolting machine drilling noise. U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2008-146 (TN 532), 2008 Sep; 1-2.

For More Information

For more information on the redesigned collapsible drill steel enclosure, contact the NIOSH Mining Program (

Page last reviewed: November 15, 2018
Page last updated: November 15, 2018