A wear bushing, or sleeve, is installed in the casing head housing, or indeed in any compact or casing head spool, to protect the hanger landing preparations from damage while the drill string, tools, or other tubulars are run through, or rotated in, the wellhead. .
In some systems, the sleeve is held in place by a set of tie-down bolts contained either in the top flange of the housing or in a separate hold-down flange installed between the housing.
[pdf] They are powered by two counter-rotating screws that compress air as it moves through the compressor. These units are designed for continuous use, capable of operating 24 hours a day, 7 days a week.
[pdf] In the application of drilling machinery, the air-leg rock drillcan only drill small-aperture, medium-shallow blast holes; the heavy-duty rail-type rock drill can also drill deep blast holes through the post, but the en.
[pdf] 
