A shear pin coupling

The invention relates to a shear pin coupling comprising at least one shear pin which, in normal operation, connects a first coupling part with a second coupling part against relative displacement in at least one direction, a plunger being provided for advancing the shear pin lengthwise, after fracture of the shear pin, into a position in which the shear pin re-establishes connection between the coupling parts, the plunger being shaped in such a way as to oppose movement of the shear pin in a direction transverse to the lengthwise direction of the shear pin.

A shear pin coupling of this kind is described in the French patent specification No. 86,925, first addition to French patent 1,402,882.

Applicant has found that a shear pin during cutting is not only subjected to force transverse to the pin, but also to force in the lengthwise direction of the pin. Due to this movement it may happen that the pin does not come back in the correct position to be sheared off a second time.

To overcome this disadvantage the plunger and the shear pin have co-operating guiding faces facing each other at an angle to the lengthwise direction of the shear pin and centralizing the shear pin with respect to the plunger.

In the NL-Patent application 74 09 104 a shear pin coupling is described, having shear pins with a specific form. The shear pin is made in form of a plunger upon which a compression spring is acting.

The present invention also provides a shear pin for use in the coupling just defined.

For a better understanding of the present invention and to show how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

• Figure 1 shows a drive shaft assembly provided with a shear pin coupling; and
• Figure 2 is a sectional view of the shear pin coupling.

The drive shaft assembly 1 of figure 1 comprises a central main part 2 connected by universal joints 3 with end parts 4 and 5 respectively. The end parts 4 and 5 are connected with a driving shaft (for example the power take-off shaft of a tractor) and with a driven shaft (for example, the input shaft of an agricultural implement) respectively. The end part 5 comprises a shear pin coupling having a first coupling part 6 and a second coupling part 7, which, in normal operation, rotate together about a rotary axis 8 (figure 2). The coupling part 6 comprises a radially extending flange 9 and a hub 10 provided with retaining means formed by a ball 11 and a screw 12 for firmly connecting the coupling part 6 axially on a stub shaft 13 of the driving or driven shaft respectively. The coupling part 7 comprises a carrier 14 and a fork 17 of the universal joint 3. The coupling part 7 is coaxial with the hub 10 and a needle bearing 15 is arranged between the carrier 14 and the hub 10. The carrier 14 comprises an inner ring 16, which is in engagement with a thrust ring which, with a retaining ring, retains the coupling part 7 axially with respect to the coupling part 6. Bolts 18 connect the carrier 14 with the fork 17, these bolts extending axially into a hard steel cutting ring 19 of the coupling part 7. On one side, the cutting ring 19 meets the flange 9 and it has a plurality of bores 20; in the embodiment shown there are ten bores. The cutting ring 19 also has two axially extending supporting bolts 21. The carrier 14 is connected near the fork 17 with a centering ring 22. The centering ring 22 and the supporting bolts 21 serve as fastening means for two holders or cassettes 23 each accommodating, in this embodiment, five shear pins 24. Each holder 23 is arcuate and is centred on the axis 8 and has at one end a plurality of apertures 25A, from which project the respective shear pins. In normal operation, one of the ten shear pins 24 projects into an aperture 9A in the flange 9, so to interconnect the two coupling parts for rotation together. The cylindrical shear pins 24 extend axially and at the end 27 away from the flange 9 they have a shape which matches a pin plunger 26 which serves as guide means for advancing the shear pin after overload into a new operative position. For this purpose the end 27 of each shear pin is conical, the vertex angle being preferably about 90°. Each pin plunger 26 has a correspondingly shaped cavity 28. Thus the shear pin and the pin plunger can fit one another at their contact surfaces. Near the shear pin the diameter of the pin plunger 26 is slightly greater than that of the shear pin, but the largest diameter of the cavity 28 substantially corresponds with the diameter of the shear pin 24. Adjacent the cavity 28 the pin plunger 26 has an annular contact surface 29, which is preferably substantially perpendicular to the conical surface of the shear pin. The contact surface 29 meets the outer circumference of the pin plunger 26, which has a shoulder 30, axially behind the surface 29, whiclf engages the inner surface of the holder 23 so as to form a centering ring for the pin plunger 26 in the holder 23. The pin plunger 26 has a cylindrical bore 31 accommodating one end of a compression spring 32, the other end of which is in contact with a cover 33 of the holder. The spring 32 presses the pin 24 against the flange 9, and presses the holder 23 with the cover 33 against the centering plate 22. The diameter of the opening 25A is chosen so that the portion of the pin plunger 26 which contacts the pin can move out of the holder 23. In order to bridge the distance between the end 25 of the holder 23 and the cutting ring 19 the axial diameter of the pin plunger 26 is chosen so that the end of the pin plunger can project far enough for the contact face 29 to come into cooperating contact with a correspondingly shaped contact face 34 located around the opening 20. The contact face 34 thus serves as a stop for the movement of the pin plunger 26.

In operation, the shear pin coupling provides protection against overload. Upon overload, the shear pin 24 interconnecting the coupling parts 6 and 7 will break, and the coupling parts 6 and 7 will then rotate relatively to one another. The tangential magnitude of the opening 9A is chosen so that the spring 32 can push the same or another one of the shear pins into the opening 9A again only when the speed of the drive has been reduced intentionally. To achieve this movement, each pin plunger 26 should guide the associated shear pin in its correct position, that is with the longitudinal axis of the shear pin parallel to the rotary axis 8. In particular, when the shear pin breaks, the remaining portion of the pin might move in a direction approximately transverse of its lengthwise direction, but this is prevented by the shape of the end of the shear pin and of the associated cavity of the pin plunger. Even after repeated breaks, the pin plunger is capable of guiding the shear pin in the manner described above to project from the holder 23. Between the holder 23 and the cutting ring 19 some axial gap is required owing to the disposition of the holder between the centering plate 22 and the supporting bolt 21. In this region between the holder and the cutting ring there is a risk for the remaining portion of the shear pin to move sideways in an undesirable manner. With the described method of guiding, the end of the shear pin is guided into direct proximity with the opening 20 so that the shear pin can be used substantially throughout its whole length without risk of damage of the pin plunger 26.

The coupling described is intended for use with 8 mm diameter pins. If desired, the cutting ring 19 can be replaced (the openings 20 being adapted to the size of the shear pins) and by using holders 23 with, for example, 9 mm diameter pins the coupling may be rendered suitable for transmitting a higher maximum torque. With this larger size the contact surfaces described above are very advantageous. The described construction of the pin plunger 26 is particularly suitable for all conventional pin diameters. If desired, the shapes of the shear pin 24 and of the pin plunger 26 may be inverted, the shear pin then having the cavity. The guide means described may also be used in other couplings than the shear pin coupling described, for example, in a stationary shear pin coupling.