Speed changer mechanism for electrically powered tool

A speed changer mechanism for an electrically powered tool having a motor shaft and an output shaft, comprising:

• a gear holder drivable by the motor shaft and having a larger-diameter gear and a smaller-diameter gear;
• a transmission shaft rotatable by said gear holder for rotating the output shaft, said transmission shaft having a splined disc mounted thereon, said splined disc having first splines:
• a high-speed gear rotably mounted on said transmission shaft and held in mesh with said larger-diameter gear, at all times, said high-speed gear having second splines;
• a low-speed gear rotatably mounted on said transmission shaft and held in mesh with said smaller-diameter gear at all times, said low-speed gear having third splines;
• a pair of springs for normally urging said high- and low-speed gears to be axially held against opposite surfaces of said splined disc;
• and
• a clutch plate having internal axial teeth and axially slidably mounted on said splined disc, said clutch plate being axially movable in a position in which said internal axial teeth mesh with said first and second splines and a position in which said internal axial teeth mesh with said first and third splines.