30 CFR § 18.34
Explosion-proof electric motor assemblies intended for use in approved
equipment in underground mines that are specifically addressed in part
of this chapter shall be approved under part
of this chapter after February 22, 1996. Those motor
assemblies not specifically addressed under part 7
chapter shall be accepted or certified under this part.
(a) General. (1) Motors shall have explosion-proof
(2) Motors submitted to MSHA for test shall be equipped
with unshielded bearings regardless of whether that type of bearing is
(3) MSHA reserves the right to test motors with the
maximum clearance specified between the shaft and the mating part which forms
the required flame-arresting path. Also reserved is the right to remachine
these parts, at the applicant's expense, to specified dimensions to provide
the maximum clearance.
NOTE: For example, a shaft with a diameter greater than 2 inches at the
flame-arresting portion might require such machining.
(4) Ball and roller bearings and oil seals will not be
acceptable as flame-arresting paths; therefore, a separate path shall be
provided between the shaft and another part, preferably in by the bearing. The
length and clearances of such flame-arresting path shall conform to the
requirements of §18.31.
(5) Labyrinths or other arrangements that provide
change(s) in direction of escaping gases will be acceptable but the use of
small detachable pieces shall not be permitted unless structurally
unavoidable. The lengths of flame-arresting path(s) and clearance(s) shall
conform to the requirements of §18.31.
(6) Oil seals shall be removed from motors prior to
submission for explosion tests.
NOTE: Oil seals will be removed from motors prior to explosion tests and
therefore may be omitted from motors submitted for investigation.
(7) Openings for filling and draining bearing lubricants
shall be so located as to prevent escape of flame through them.
(8) An outer bearing cap will not be considered as forming
any part of a flame-arresting path unless the cap is used as a bearing
NOTE: The outer bearing cap will be omitted during explosion tests unless it
houses the bearing.
(9) If unavoidable, holes may be made through motor
casings for bolts, studs, or screws to hold essential parts such as pole
pieces, brush rigging, and bearing cartridges. Such parts shall be attached
to the casing by at least two fastenings. The threaded holes in these parts
shall be blind, unless the fastenings are inserted from the inside, in which
case the fastenings shall not be accessible with the armature of the motor
(b) Direct-current motors. For direct-current motors with
narrow interpoles, the distance from the edge of the pole piece to any bolt
hole in the frame shall be not less than 1/8 inch. If the distance is 1/8 to
1/4 inch, the diametrical clearance for the pole bolt shall not exceed 1/64
inch for not less than 1/2 inch through the frame. Furthermore, the pole
piece shall have the same radius as the inner surface of the frame. Pole
pieces may be shimmed as necessary.
(c) Alternating-current motors. Stator laminations that form
a part of an explosion-proof enclosure will be acceptable provided: (1) The
laminations and their end rings are fastened together under pressure; (2)
the joint between the end rings and the laminations is not less than 1/4
inch, but preferably as close to 1 inch as possible; and (3) it shall be
impossible to insert a 0.0015-inch thickness gage to a depth exceeding 1/8
inch between adjacent laminations or between end rings and laminations.
(d) Small motors (alternating- and direct-current). Motors
having internal free volume not exceeding 350 cubic inches and joints not
exceeding 32 inches in outer circumference will be acceptable for
investigation if provided with rabbet joints between the stator frame and the
end bracket having the following dimensions:
DIMENSIONS OF RABBET JOINTS--INCHES
| | | | Max. |
| | Min. Width | Max. | diametrical |
| Minimum total width | of clamped | clearance | clearance |
| | radial | of radial | at axial |
| | portion | portion | portion |
| 3/8.................| 3/64 | 0.0015 | 0.003 |
| 1/2.................| 3/64 | .002 | .003 |
| 1/2.................| 3/32 | .002 | .004 |