Power tool trigger assembly

The present invention relates to a trigger assembly for an electric power tool.

BACKGROUND OF THE INVENTION

The operation of electric hand drills is often controlled by means of a pull-trigger, which is used to switch on and off the motor as well as to adjust its speed/torque. Certain auxiliary electronic/electrical devices, such as battery and level meters, may be preferred. It will be convenient if such devices can also be controlled using the pull-trigger.

The invention seeks to provide a new trigger assembly that can readily be used to control the operation of such auxiliary devices.

SUMMARY OF THE INVENTION

According to the invention, there is provided a trigger assembly for controlling the operation of an electric power tool including an electric motor, comprising a base for fixing within the power tool and a trigger supported by the base for inward and outward movement. The base comprises a housing, a first mechanical switch provided in the housing and operable to switch on and off the motor, and an electronic operating circuitry provided in the housing for controlling the operation of the motor when the first switch is closed. The base includes a slider guided within the housing for movement by or with the trigger to operate the first switch and the operating circuitry. Also included is a second mechanical switch provided within the housing and operable in response to the movement of the trigger to switch on and off an auxiliary device for the power tool. The auxiliary device is located externally of the base and is operable independently of the operating circuitry.

Preferably, the trigger is resiliently biassed towards a foremost home position, and the second switch is open when the trigger is in the home position and will be closed when the trigger is moved away from the home position.

In a preferred embodiment, the second switch comprises a stationary contact and a movable contact resiliently biassed towards the stationary contact.

More preferably, the trigger is resiliently biassed towards a foremost home position, and the movable contact of the second switch is moved out of contact from the stationary contact by the slider while the trigger is in the home position.

More preferably, the movable contact of the second switch is engageable by the slider against the action of resilience.

Further more preferably, the slider has a front bottom part for abutting the movable contact of the second switch.

More preferably, the movable contact of the second switch is cantilevered for pivotal movement.

More preferably, one of the contacts of the second switch is connected to positive electrical connection of the operating circuitry.

It is preferred that the first and second switches are operable by respective first and second parts of the slider that are adjacent each other, the first part having a surface for slidably engaging a moving contact of the first switch.

It is further preferred that the surface is discontinuous to minimise the risk of breakdown or flashover therealong.

It is yet further preferred that the surface includes a groove.

In a specific construction, the housing has an upper portion housing the slider and a lower portion including a front cavity housing the second switch.

More specifically, the housing has a frontmost outer wall and an inner wall behind the outer wall, the two walls defining the cavity.

The invention also provides an electric power tool including an electric motor and the aforesaid trigger assembly.

In one example, the electric power tool is an electric hand drill.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1

is a simplified circuit diagram of an embodiment of a trigger assembly in accordance with the invention, connected to an electric power tool incorporating a motor;

FIG. 2

is a right side view of the trigger assembly of

FIG. 1

, partially broken to reveal a built-in switch thereof;

FIG. 3

is a front view of the trigger assembly of

FIG. 2

;

FIG. 4

is a right side perspective view of the trigger assembly of

FIG. 2

, with a moving contact of the built-in switch disassembled;

FIG. 4A

is an enlarged part of

FIG. 4

as encircled;

FIG. 5

is a right side perspective view corresponding to

FIG. 4

, showing the moving contact assembled;

FIG. 5A

is an enlarged part of

FIG. 5

as encircled;

FIG. 6

is a right side internal view of the trigger assembly of

FIG. 2

, showing the built-in switch in an open condition;

FIG. 6A

is an enlarged part of

FIG. 6

as encircled;

FIG. 7

is a right side internal view corresponding to

FIG. 6

, showing the built-in switch in a closed condition;

FIG. 7A

is an enlarged part of

FIG. 7

as encircled;

FIG. 8

is a perspective view of the built-in switch of

FIG. 7

, together with a slider for operating it; and

FIG. 9

is a perspective view of the slider of

FIG. 8

, showing it in greater detail.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring to the drawings, there is shown a trigger assembly

100

embodying the invention for controlling the operation of an electric power tool such as a hand drill

10

that incorporates an electric motor

12

. The trigger assembly

100

comprises an upright generally rectangular base

200

fixed inside the drill body adjacent the upper end of its handgrip and a pull-trigger

300

supported by the base

200

for inward (rearward) and outward (forward) horizontal sliding movement. A housing

210

of the base

200

has an upper portion

212

from inside which the pull-trigger

300

extends forwards, and a lower portion

214

that includes a frontmost vertical outer wall

216

and an inner wall

218

at a small distance behind the outer wall

216

. The two walls

216

and

218

define a narrow cavity

220

housing a mechanical built-in switch

500

.

The trigger assembly

100

incorporates, as contained within its base housing

210

, an electronic operating circuitry

400

which comprises an IC control circuit

410

and a solid-state switch

420

controlled by the circuit

410

. A mechanical main switch

430

operated by the pull-trigger

300

is also contained within the housing

210

. The two switches

420

and

430

are connected in series with the motor

12

across positive (Vcc) and negative (GND) terminals

440

of a rechargeable DC battery pack for the hand drill

10

. In use, the main switch

430

switches on the motor

12

upon (or shortly after) pulling back of the pull-trigger

300

and later switches it off when the pull-trigger

300

returns to its initial foremost (outermost) home position. While the main switch

430

is closed, the solid-state switch

420

controls the operation of the motor

12

.

The control circuit

410

comes into operation upon closing of the main switch

430

, whereupon it triggers the solid-state switch

420

to switch on and off at a relatively high frequency having a variable duty cycle according to the travelling position of the pull-trigger

300

for adjusting the speed/torque of the motor

12

. A flywheel diode

450

and a double-pole double-throw reversal switch

460

are connected across the terminals of the motor

12

. The flywheel diode

450

is connected to the motor

12

by the reversal switch

460

only when the reversal switch

460

switches the motor

12

to run in the forward direction. The flywheel diode

450

allows the motor current to continue to flow while the solid-state switch

420

is non-conducting while switching.

Another mechanical, bypass switch

435

, also operated by the pull-trigger

300

, is connected in parallel with the main and solid-state switches

430

and

420

for bypassing them, which is connected from the negative (GND) battery terminal

440

to beyond the solid-state switch

420

. The bypass switch

435

will be closed when the pull-trigger

300

is (almost) fully pulled back for bypassing the solid-state switch

420

, thereby delivering full power directly to the motor

12

for maximum speed/torque operation.

The pull-trigger

300

has a body

310

exposed for manual pulling, a generally rectangular core slider

320

guided within the upper portion

212

of the base housing

210

for back and forth sliding movement, and a horizontal shaft

330

interconnecting the trigger body

310

and slider

320

for simultaneous movement. The core slider

320

, which is resiliently biassed forwards from behind by a compression coil spring

322

, carries on its right side a four-pronged sliding contact

324

. The contact

324

bears slidably against a row of contact strips on a circuit board mounting the control circuit

410

for selectively making contact therewith, as the slider

320

is pushed inwards by the trigger body

310

or outwards by the spring

322

upon release of the trigger body

310

. The sliding position of the contact

324

determines the duty cycle of the trigger signal generated by the control circuit

410

for switching on and off the solid-state switch

420

.

The travel of the core slider

320

is limited by opposite front and rear ends of the interior of the upper portion

212

of the base housing

210

. Under the action of the spring

322

, the slider

320

normally stays foremost, bearing against an upper end of the housing wall

216

. The slider

320

has a shallow end recess

326

at its front bottom corner on the right side and adjacent the upper end of the wall

216

, and includes a pair of cams on its bottom surface for operating the main and bypass switches

430

and

435

. The lower surface of the cams taken as a whole, over a relatively short region thereof adjacent or leading to the end recess

326

, is formed with a series of three grooves

328

(FIG.

9

). The grooves

328

run transversely (or at an acute angle) across the complete width of this region, thereby interrupting the surface to render it discontinuous or lengthen its surface length over this region.

The detailed construction and operation of the main and bypass switches

430

and

435

are described in a related utility patent application Ser. No. 10/443,060 entitled “Power Tool Trigger Assembly” filed on the same day in the name of the same inventors, the disclosure thereof is hereby incorporated by reference. In particular, the main and bypass switches

430

and

435

make use of respective contact levers as moving contacts that are pivotable through sliding engagement by the aforesaid cams of the core slider

320

for making and breaking contact.

The built-in switch

500

is formed by a rigid stationary contact bar

510

and a resilient movable contact lever

520

positioned right in front of the contact bar

510

for flexing into contact therewith or flexing out of contact therefrom, to perform a switching action. The contact lever

520

is biassed towards or against the contact bar

510

by virtue of self-resilience.

The contact bar

510

has a flat middle section

512

for contact by the contact lever

520

, a crooked upper end

514

extending laterally across the core slider

320

and connected to positive electrical connection (Vcc) of the control circuit

410

, and a 90°-turned lower end

516

. The middle section

512

is housed within the cavity

220

, whilst the upper and lower ends

514

and

516

emerge through opposite ends of the cavity

220

. Two sharp ribs

513

on the surface of the middle section

512

assist good contact making. A side lug

515

of the lower end

516

is formed with a hole for fixing of the bar

510

.

The contact lever

520

has an arcuate middle section

522

for contacting the contact bar

510

, an upper end

524

engageable by the core slider

320

, and a lower end

526

fixed to the lower end of the cavity

220

. The lever

520

is cantilevered for pivotal movement about its lower end

526

, with its upper end

524

capable of bearing backwards resiliently against the corner recess

326

of the core slider

320

. The lower end

526

includes a side lug

525

, to which an electric cable

528

is connected for external electrical connection.

While the pull-trigger

300

is in the home position, its core slider

320

abuts and thus blocks the contact lever

520

against contacting the contact bar

510

, whereby the built-in switch

500

is normally open (FIGS.

6

and

6

A). Upon pulling back of the trigger body

310

, the slider

320

retreats and thus allows the contact lever

520

to move towards the contact bar

510

. After the slider

320

has slid back to a threshold position at a certain small distance from its foremost position, the lever

520

comes into contact with the bar

510

, engaging by their middle sections

522

and

512

, whereby the switch

500

is closed (FIGS.

7

and

7

A). The switch

500

will remain closed for as long as the slider

320

is pressed backwards beyond the threshold position, until the slider

320

is released and returns forwards past the threshold position.

The switch

500

is installed inside the trigger assembly

100

and more specifically its base

200

for switching on and off an auxiliary electrical/electronic device for the hand drill, such as a battery meter or level meter. Such an auxiliary device is located externally of the trigger assembly

100

and normally on the drill body, and is operable independently of the motor driving circuitry.

In this particular embodiment, whilst the stationary switch contact

510

is connected to the positive terminal (Vcc) of the DC battery pack, the movable switch contact

520

is connected via the cable

528

to the external auxiliary device. The built-in switch

500

serves to switch on and off the power to the auxiliary device, such that the device will be automatically turned on (or enabled) upon pressing of the trigger assembly

100

.

The external auxiliary device draws a much smaller current and is more delicate than the motor

12

. Due to the compact. design of the trigger assembly

100

, the built-in switch

500

has to be situated very close to the main/bypass switches

430

/

435

as in the case of the described embodiment. In particular, the built-in switch

500

shares the same operator, i.e. the core slider

320

, as the main/bypass switches

430

/

435

.

During operation, as the contact levers of the main/bypass switches

430

/

435

always bear and rub against the cams of the slider

320

, a small amount of their conductive material will in the course of time be transferred to the cam surface. As the material builds up on the cam surface particularly over the aforesaid region adjacent or leading to the end recess

326

, a conductive surface path will inevitably be formed. In an extreme or faulty condition, the conductive path is prone to breakdown or flashover between the conducting parts of the main/bypass switches

430

/

435

and built-in switch

500

, thereby damaging the auxiliary device. The grooves

328

minimise the risk of such breakdown or flashover by interrupting the surface of this path or extending its surface length.

The subject trigger assembly may be utilized to control any other types of electric power tools, such as a reamer, cutter or saw.

The invention has been given by way of example only, and various other modifications of and/or alterations to the described embodiment may be made by persons skilled in the art without departing from the scope of the invention as specified in the appended claims.