Speed changer mechanism for electrically powered tool

An electrically powered tool such as a drill has a gear holder drivable by a motor shaft and having a larger-diameter gear and a smaller-diameter gear, a transmission shaft rotatable by the gear holder for rotating an output shaft, the transmission shaft having a splined disc having first splines, a high-speed gear rotatably mounted on the transmission shaft and held in mesh with the larger-diameter gear, the high-speed gear having second splines, a low-speed gear rotatably mounted on the transmission shaft and held in mesh with the smaller-diameter gear, the low-speed gear having third splines, a pair of springs for normally urging the high- and low-speed gears to be axially held against opposite surfaces of the splined disc, and a clutch plate having internal axial teeth and axially slidably mounted on the splined disc. The clutch plate is axially movable in a position in which the internal axial teeth mesh with the first and second splines and a position in which the internal axial teeth mesh with the first and third splines.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a speed changer mechanism for use in an 
electrically powered tool such as a drill. 
2. Prior Art 
Some electrically powered tools have a speed changer mechanism for rotating 
a tool such as a drill bit selectively at a high speed or a low speed. 
Typically, such a speed changer mechanism comprises a pair of high- and 
low-speed gears rotatably mounted on an output shaft and held in mesh with 
larger- and smaller-diameter gears, respectively, on a transmission shaft 
which is rotatable by a motor. Between the high- and low-speed gears that 
are axially spaced from each other, there is a dog clutch plate axially 
movably splined to the output shaft. The dog clutch plate can be axially 
moved by a shift knob which is manually rotated by the user of the tool. 
When a low-speed mode of operation is selected, the shift knob is rotated 
to axially move the dog clutch plate in one direction until teeth thereon 
fit into recesses defined in one side of the low-speed gear. Now, rotation 
of the transmission shaft is transmitted from the smaller-diameter gear 
thereon through the low-speed gear and the dog clutch plate to the output 
shaft which then rotates a tool coupled thereto. In a high-speed mode of 
operation, the dog clutch plate is axially moved in the opposite direction 
to fit its teeth into recesses defined in one side of the high-speed gear. 
The output shaft is then rotated by the transmission shaft through the 
larger-diameter gear, the high-speed gear, and the dog clutch plate. 
The high- and low-speed gears are normally urged by respective springs so 
as to be pressed against a cylindrical member of the shift knob which 
engages a peripheral surface of the dog clutch plate. However, the high- 
and low-speed gears tend to undergo localized loads because their 
peripheral edge portions are pressed against the cylindrical member of the 
shift knob. Accordingly, the high- and low-speed gears are liable to 
generate noise during operation, and have a short service life due to 
localized wear thereof. 
Another problem is that the high- and low-speed gears and the dog clutch 
plate are required to be large and heavy to transmit the torque from the 
transmission shaft to the output shaft. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a speed changer 
mechanism for an electrically powered tool, which includes high- and 
low-speed gears that are substantially free of localized loads and do not 
produce large noise. 
Another object of the present invention is to provide a speed changer 
mechanism for an electrically powered tool, which is small in size and 
light in weight. 
According to the present invention, there is provided a speed changer 
mechanism for a 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 the gear holder for rotating the output shaft, the 
transmission shaft having a splined disc having first splines, a 
high-speed gear rotatably mounted on the transmission shaft and held in 
mesh with the larger-diameter gear, the high-speed gear having second 
splines, a low-speed gear rotatably mounted on the transmission shaft and 
held in mesh with the smaller-diameter gear, the low-speed gear having 
third splines, a pair of springs for normally urging the high- and 
low-speed gears to be axially held against opposite surfaces of the 
splined disc, and a clutch plate having internal axial teeth and axially 
slidably mounted on the splined disc, the clutch plate being axially 
movable in a position in which the internal axial teeth mesh with the 
first and second splines and a position in which the internal axial teeth 
mesh with the first an third splines. 
The gear holder is axially coupled between and supported on the motor shaft 
and the output shaft. 
The high- and low-speed gears have smaller-diameter portions, respectively, 
the second and third splines being disposed on the smaller-diameter 
portions, respectively 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description when 
taken in conjunction with the accompanying drawing in which a preferred 
embodiment of the present invention is shown by way of illustrative 
example.

DETAILED DESCRIPTION 
As shown in the sole FIGURE, an electrically powered tool 10, typically a 
drill, which incorporates a speed changer mechanism according to the 
present invention, has a casing 11 housing a motor 12 having a rotatable 
motor shaft 13 on which a planetary gear mechanism 14 is mounted. The 
motor shaft 13 also supports a gear holder 15 having a larger-diameter 
gear 16 and a smaller-diameter gear 17. The larger- and smaller-diameter 
gears 16, 17 are held in mesh with a smaller-diameter high-speed gear 18 
and a larger-diameter low-speed gear 19, respectively. The high- and 
low-speed gears 18, 19 are axially spaced from each other and rotatably 
supported on an idler transmission shaft 20 which is rotatably supported 
in the casing 11 bearings 21, 22. 
A clutch mechanism 23, which is disposed axially between the high- and 
low-speed gears 18, 19, includes an axially slidable clutch plate 24 
having internal axial teeth. The transmission shaft 20 has a 
larger-diameter splined disc 25 positioned axially between the high- and 
low-speed gears 18, 19 and having splines 26 on its outer peripheral 
surface. The high- and low-speed gears 18, 19 have smaller-diameter 
portions projecting inwardly toward the splined disc 25 and having splines 
27, 28 on their outer pheripheral surfaces. The splines 26, 27, 28 are 
axially adjoined and radially positioned on the same circular surface. The 
clutch plate 24 is disposed around the splined disc 25 and the 
smaller-diameter portions of the high- and low-speed gears 18, 19, with 
the internal axial teeth of the clutch plate 24 meshing with the splines 
26, 27, 28. The axial dimension of the clutch plate 24 is such that the 
clutch plate 24 can mesh with the splines 26, 27 or the splines 27, 28, 
but not with all splines 26, 27, 28 simultaneously. The clutch plate 24 is 
axially movable by a shift knob (not shown) which can be operated by the 
user of the tool 10. 
The high- and low-speed gears 18, 19 are normally urged so as to be held 
against the opposite sides of the splined disc 25 by respective helical 
compression springs 29 30 which are disposed under compression between the 
high-speed speed gear 18 and the bearing 21 and between the low-speed gear 
19 and the bearing 22. 
The tool 10 has an output shaft 31 rotatably supported in the causing 11 by 
bearings 32, 33. The output shaft 31 is operatively coupled to the 
transmission shaft 20 through a gear 34 on one end of the transmission 
shaft 20 and a gear 35 on one end of the output shaft, the gears 34, 35 
meshing with each other. The rotation of the output shaft 31 can be 
controlled by a clutch mechanism 36 disposed between the output shaft 31 
and the gear 35. A tool bit such as a drill bit (not shown) is axially 
connected to the output shaft 31 by a chuck 37. 
The gear holder 15 is axially coupled between and supported by the motor 
shaft 13 and the output shaft 31. 
The tool 10 thus constructed operates as follows: 
When the tool 10 is to operate in a high speed mode, the clutch plate 24 is 
axially moved to the left as shown to connect the high-speed gear 18 to 
the transmission shaft 20 through the splines 26, 27. Then, the motor 12 
is energized. The rotation of the motor 12 is transmitted from the motor 
shaft 13 through the planetary gear mechanism 14, the larger-diameter gear 
16, the high-speed gear 18, and the clutch mechanism 23 to the 
transmission shaft 20. The transmission shaft 20 then rotates the output 
shaft 31 through the intermeshing gears 34, 35 at a high speed. 
To operate the tool 10 in a low speed mode, the clutch plate 24 is axially 
moved to the right to connect the low-speed gear 19 to the transmission 
shaft 20 through the splines 26, 28. Then, the motor 12 is energized. The 
rotation of the motor 12 is transmitted from the motor shaft 13 through 
the planetary gear mechanism 14, the smaller-diameter gear 17, the 
low-speed gear 19, and the clutch mechanism 23 to the transmission shaft 
18. The transmission shaft 18 then rotates the output shaft 31 through the 
intermeshing gears 34, 35 at a low speed. 
Even if the clutch plate 24 does not immediately mesh the splines 27 or 28 
when it is axially moved, the clutch plate 24 will be brought into mesh 
with the splines 27 or 28 under repulsive forces of the helical 
compression springs 29, 30 when the motor shaft 13 is rotated. Therefore, 
the clutch plate 24 can smoothly mesh with the splines 27 or 28 at all 
time irrespective of whether the high or low speed mode is selected. The 
speed changer mechanism of the present invention is thus smooth in 
operation. 
Since the high- and low-speed gears 18, 19 are held at their surfaces 
around and near the axes thereof against the opposite sides of the splined 
disc 25, the high- and low-speed gears 18, 19 are free of localized loads 
and wear due to the compressive forces from the helical compression 
springs 29, 30. Accordingly, the high- and low-speed gears 18, 19 are held 
in smooth mesh with the larger- and smaller-diameter gears 16, 17, 
respectively, do not produce large noise, rotate smoothly, and have a long 
service life. 
Inasmuch as the high- and low-speed gears 18, 19 and the clutch plate 25 
are mounted on the transmission shaft 20, rather than the output shaft 31, 
they may be small in size and light in weight. The gear holder 15 which 
has the two gears 16, 17 is axially connected between and supported by the 
motor shaft 13 and the output shaft 31, so that the tool 10 is compact in 
size. 
Although a certain preferred embodiment has been shown and described, it 
should be understood that many changes and modifications may be made 
therein without departing from the scope of the appended claims.