A spring member extending from a first end to a second end about a longitudinal axis, the spring having an inherent bias to assume an extended position with a first end spaced from the second end along the axis, the spring assuming compressed positions when compressed by forces applied parallel to the axis, in the compressed positions the spring resiliently urges its first and second ends axially away from each other toward the extended position, the spring member having a wall in the shape of a solid of revolution rotated about the axis and defining a central cavity therein open at the first end of the spring and substantially closed at the second end of the spring, the wall when in the unbiased extended position having a greatest diameter at the first end and a least diameter at the second end, a plurality of openings through the wall, the openings disposed symmetrically both circumferentially and axially relative to each other.