GB2077151A - Rotary drive for a hammer drill - Google Patents

Abstract

The drive has a sleeve 4 to which the drill bit is coupled. The rotary drive of the sleeve includes a drive gear 3 and a friction clutch formed by a set of gears 5 which mesh with the drive gear and are axially movable and rotatable relative to the sleeve 4. The clutch also has friction plates 6 and 11 which are axially movable along and rotationally fixed relative to the sleeve. The opposite outer plates 6 and 11 are biased by means of a nut 7 and a spring 8. The nut 7 has on the side directly engaging the outer friction plate 11 holes 9 for accommodation of balls 10 and the friction plate 11 engaging the nut 7 has corresponding recesses. The balls are received in the nut holes 9 and plate recesses 12 with axial play. This is less than the depth at which the balls 10 enter the recesses 12 or the holes 9. For adjustment the nut is rotated relative to the sleeve. This initially increases the spring loading. The loading decreases to the new value when the balls 10 drop into the successive plate recesses 12. The biassing is on the unloading curve of the hysteresis curve of the clutch. <IMAGE>

Description

SPECIFICATION Rotary drive for a drill in a rotary hammer The present invention relates to rotary hammers, and more particularly, to rotary drives for a drill in rotary hammers to be used in the construction and mining industries and in geological prospecting.

According to the invention a rotary drive for a drill in a rotary hammer, wherein a drill holder is connected with a sleeve which is caused to rotate by means of a drive, including a drive gear and a friction clutch which transmits rotary motion to the sleeve and consists of a set of gears which mesh with the drive gear and are axially and radially movable on the sleeve, and friction plates which are axially movable along and radially fixed relative to the sleeve and elastically biased by means of a nut and a spring providing an axial bias force, the nut has holes for accommodatoing of balls on the side directly engaging the outer friction plate, and the friction plate engaging the nut has recesses equally spaced along a circle of a radius which is equal to the radius of the circle along which the nut holes are spaced, the balls being received in the nut holes and respective recesses of the friction plate with an axial play which is smaller than the depth at which the balls enter the recesses of the friction plate or the holes of the nut.

The advantage of such construction resides in that the force determining the torque transmitted by the friction clutch is set on the unloading branch of a hysteresis curve representing the mechanical characteristic of the clutch. Dynamic loads acting on the clutch in operation cannot thus cause any drop of torque transmitted through the clutch.

The invention will now be described with reference to the accompanying drawings illustrating a rotary drive for a drill in a rotary hammer according to the invention, in which: Figure 1 is a sectional view of a rotary drive for a drill in a rotary hammer; Figure 2 shows the position of a ball in recesses of a friction plate and in holes of a nut; Figure 3 is a diagram showing the loading mechanical characteristic, wherein f is spring deformation, P is force acting on the spring.

A rotary drive for a drill in a rotary hammer shown in Fig. 1 comprises a casing 1 accommodating a drive shaft 2 having a drive gear 3 fitted thereon. The rotary drive of the rotary hammer also has a protective friction type clutch which consists of a sleeve 4, drive friction plates 5 comprising gears which mesh with the drive gear and are axially and radially movable on the sleeve 4, driven plates 6 installed alternately with the plates 5, which are axially movable along and radially fixed relative to the sleeve. The friction plates are biased by a shoulder of the sleeve 4 and a nut 7 by means of a Belville spring 8. As best shown in Fig. 2, the nut 7 has holes 9 for accommodation of balls 10 and a friction plate 11 adjacent to the nut 7 has recesses 12 equally spaced along a circle, which receive the balls 10 when the recesses 12 and the holes 9 are in register.The sleeve 4 is coupled to a drill holder 13 in which a drill 14 is inserted. The balls 10 are received in the holes 9 of the nut 7 and in the recesses 12 of the plate with an axial play which is smaller than the depth at which the balls enter the recesses 12 or holes 9 so that the balls slightly protrude over the face of the nut 7. In order that the holes 9 of the nut 7 could be brought in register with the recesses 12 of the plate 11, the radius of the circle along which the recesses 12 are spaced is equal to the radius of a circle along which the holes 9 are arranged in the nut 7.

The device according to the invention functions in the following manner.

When the drive shaft 2 rotates, the drive gear 3 transmits rotary motion to the drive friction plates 5. The plates 6 and 11 rotate together with the plates 5 owing to frictional engagement to transmit rotary motion to the sleeve 4 and thereform to the holder 13 of the drill 14. In case a torque set-up at the clutch is overcome, e.g. when the drill 14 is jammed resulting in a stall of the drill holder, sleeve 4 and plates 6 and 11, the plates 5 slip relative to the plates 6 and 11 thereby limiting the load applied to the hands of the operator.

Stability of the torque transmitted by the protective clutch, hence an improved reliability of the rotary drive for a drill is obtained owing to the above-described arrangement of the balls which enables the setting of the bias force applied to the friction plates 6. 11 and 5 on the unloading branch a-b-d-0 of a hysteresis curve (Fig. 3) which represents the mechanical loading characteristics of the clutch. The bias force applied to the friction plates 6, 11 and 5 determines the torque transmitted by the protective clutch and is setup by turning the nut 7 relative to the sleeve 4.Rotation of the nut 7 relative to the sleeve 4 is accompanied by the balls 10 being forced out from the recesses 12 of the plate 11 and compression of the spring 8 with a force P3 which is greater (to an extent depending on the amount of protrusion of the balls 10 over the nut 7) than that required to set a desired torque (point d of the loading curve c-a in Fig. 3). After the nut 7 is turned at an angle equal to the spacing of the recesses 12 in the plate 11 the balls 10 enter the adjacent recesses 12 to relieve the spring 8 which is now compressed with a force P2 (point b of the unloading curve a-b in Fig. 3).The difference between the forces P2 and P1 is an additional load applied to the spring 8 when the nut 7 is turned at an angle corresponding to the angular distance between two adjacent recesses 12 of the plate 11. The torque at the clutch set-up at the compression force P2 at point b of the unloading curve a-b in Fig. 3 is retained under the action of dynamic loads since a short-time increase in the compression force of the spring 8 occurs along the loading curve b-a with subsequent return back to the point b along the unloading curve a-b (see Fig. 3).

It can be seen in Fig. 3 that in prior art devices the force was set along the loading branch (curve e-a). The pre-set torque transmitted through the clutch was set under a certain load P2 at point e on the rising branch ~-a of the hysteresis curve in Fig. 3. When the clutch was under a dynamic load, e.g.

when a hammer piston hitted thereagainst, it oscillated, and the spring underwent a shorttime compression owing to the inertia of the set of friction plates to a force P3 at point a of the rising branch O-a in Fig. 3. After the inertia forces ceased to act the spring expanded again to a deformation f equal to the initial precompression of the spring. Expansion of the spring and return to the initial deformation f of the spring occured along the downfalling branch a-d of the hysteresis curve (shown in dotted lines) to the point d in Fig. 3.

At this point, as shown in the drawing, a force p5 acted which was smaller than the initial force P2 so that the torque transmitted by the clutch became lower.

Comparison of the torque response of the clutch when set-up along the loading curve O-e-a and along the unloading curve a-b-c-d-0 shows that the device according to the invention ensures the stability of torque transmitted by the clutch under the action of dynamic loads.

Claims (3)

1. A rotary drive for a drill in a rotary hammer, wherein a drill holder is coupled to a sleeve which is caused to rotate by means of a rotary drive, including a drive gear, a friction clutch transmitting rotary motion to the sleeve and consisting of a set of gears which mesh with said drive gear and are axially and radially movable on the sleeve, and friction plates which are axially movable along and radially fixed relative to the sleeve, the outer opposite friction plates being elastically biased by means of a nut and a spring providing an axial bias force, said nut having holes on the side directly engaging the outer friction plate to accommodate balls and the friction plate engaging the nut having recesses equally spaced along a circle of a radius which is equal to the radius of a circle along which are spaced the holes of the nut, the balls being received in the nut holes and respective recesses of the friction plate with an axial play which is smaller than the depth at which the balls enter the recesses of the friction plate of the holes of the nut.

2. A rotary drive as claimed in Claim 1, wherein the nut and the spring are arranged at opposite end friction plates, respectively.

3. A rotary drive for a drill in a rotary hammer substantially as hereinabove described with reference to, and as shown in the accompanying drawings.