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A transmission housing for a hammer drill comprises two clamshell halves and that mate to define a first transmission housing chamber and a second transmission housing chamber disposed on either side of a ball bearing race. The first and second transmission housing chambers are interconnected by cha

A transmission housing for a hammer drill comprises two clamshell halves and that mate to define a first transmission housing chamber and a second transmission housing chamber disposed on either side of a ball bearing race. The first and second transmission housing chambers are interconnected by channels. The rear end of a hollow piston, a cylindrical bearing, a crank pin and a crank plate are disposed in the first transmission housing chamber, and a spindle is disposed in the second transmission housing chamber. Overflow channels and allow movement of air between the first and second transmission housing chambers to reduce compression of air iduring reciprocation of the hollow piston.

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The invention claimed is: 1. A power tool comprising: an outer housing for gripping by a user; a motor disposed in the outer housing and including an output shaft; a transmission mechanism including: a hollow cylinder; a piston mounted for reciprocating motion within the cylinder; a ram mounted fo

The invention claimed is: 1. A power tool comprising: an outer housing for gripping by a user; a motor disposed in the outer housing and including an output shaft; a transmission mechanism including: a hollow cylinder; a piston mounted for reciprocating motion within the cylinder; a ram mounted for reciprocating motion within the cylinder and located forward of the piston; and a space defined within the cylinder and between the piston and the ram and serving as an air spring for transmitting the reciprocating motion of the piston to the ram, a transmission housing located within the outer housing and substantially enclosing the transmission mechanism the transmission housing including an interior support structure for supporting the hollow cylinder within the transmission housing, and wherein the transmission housing, the hollow cylinder, and the piston substantially define a first chamber rearward of the support structure; and the transmission housing, the hollow cylinder, and the ram substantially define a second chamber forward of the support structure and separate from the first chamber; and a channel pneumatically connecting the first chamber and the second chamber in order to allow air flow between the first chamber and the second chamber to reduce pressure differences generated by the reciprocation of the piston within the first chamber. 2. A power tool according to claim 1, wherein the transmission housing is formed from a plurality of clamshell portions. 3. A power tool according to claim 2, wherein at least one clamshell portion defines the channel. 4. A power tool according to claim 1, further comprising a plurality of internal guide rails on which the transmission housing is slidably mounted within the outer housing. 5. A power tool according to claim 1, further comprising biasing means for damping vibrations of the transmission mechanism transmitted to the outer housing. 6. A power tool according to claim 5, wherein the biasing means comprises at least one coil spring. 7. A power tool according to claim 1, wherein the power tool is a hammer drill. 8. A power tool comprising: an outer housing for gripping by a user; a motor disposed in the outer housing and including an output shaft; a transmission mechanism including: a hollow cylinder; a hollow piston mounted for reciprocating motion within the cylinder; a ram mounted for reciprocating motion within the hollow piston; and a space defined by the hollow piston and the ram and serving as an air spring for transmitting the reciprocating motion of the piston to the ram, a transmission housing located within the outer housing and substantially enclosing the transmission mechanism the transmission housing including an interior support structure for supporting the hollow cylinder within the transmission housing, and wherein the transmission housing, the hollow cylinder, and the piston substantially define a first chamber rearward of the support structure; and the transmission housing, the hollow cylinder, and the ram substantially define a second chamber forward of the support structure and separate from the first chamber; and a channel pneumatically connecting the first chamber and the second chamber in order to allow air flow between the first chamber and the second chamber to reduce pressure differences generated by the reciprocation of the piston within the first chamber. 9. A power tool according to claim 8, wherein the transmission housing is formed from a plurality of clamshell portions. 10. A power tool according to claim 9, wherein at least one clamshell portion defines the channel. 11. A power tool according to claim 8, further comprising a plurality of internal guide rails on which the transmission housing is slidably mounted within the outer housing. 12. A power tool according to claim 8, further comprising biasing means for damping vibrations of the transmission mechanism transmitted to the outer housing. 13. A power tool according to claim 12, wherein the biasing means comprises at least one coil spring. 14. A power tool according to claim 8, wherein the hollow cylinder is an output spindle rotatably mounted within the transmission housing. 15. A power tool according to claim 14, wherein the support structure includes a bearing race located circumferentially around the output spindle and rotatably supporting the output spindle. 16. A power tool according to claim 14, wherein the output spindle defines an interior bore and the ram reciprocates into the interior bore. 17. A power tool according to claim 16, wherein the output spindle defines a vent hole pneumatically connecting the interior bore and the second chamber. 18. A power tool according to claim 14, wherein the transmission mechanism is a first reciprocating transmission mechanism and the power tool further comprises a second rotary transmission mechanism substantially located within the transmission housing for selectably driving the output spindle in rotation. 19. A power tool according to claim 18, wherein the second rotary transmission includes a driven gear located circumferentially around the output spindle between the first chamber and the second chamber. 20. A power tool according to claim 19, wherein the channel is defined by the transmission housing. 21. A power tool according to claim 20, wherein the transmission housing is formed from a plurality of clamshell portions.