Joint forming device

Details Joint forming device Joint forming device

1999-05-14

2001-08-21

Kim, Harry C. (Department: 3629)

Joints and connections

Having a cam, wedge, or tapered portion

C403S231000

Reexamination Certificate

active

06276868

ABSTRACT:

The invention relates to joint forming devices and particularly, but not exclusively, to joint forming devices for forming a joint between two members of an item of knock-down furniture.
The applicant's U.S. Pat. No. 5,567,081 discloses a joint forming device for forming a joint between two members. Referring to
FIG. 1
of the drawings of the present application, the known joint forming device comprises an elongate fastening element
10
and a rotatable tightening element
12
. The fastening element
10
is fixable to a first (
14
) of the joint members such that at least a head portion
16
thereof protrudes from the joint member. The tightening element
12
is rotatably mounted in a recess
18
in the second
20
of the joint members and has two arcuate camming surfaces
22
which are engageable with the head portion
16
of the fastening element.
The joint forming device further comprises a sleeve
24
for the fastening element
10
. One end of the sleeve
24
is fittable into a recess
26
in the first joint member
14
. The fastening element
12
has a multi-tapered end portion
28
which is arranged to cause the sleeve to expand and forcibly engage a wall
30
of the recess
26
when the fastening element is moved axially relative to the sleeve.
In use, the sleeve
24
is fitted to the fastening element
10
and then inserted into a bore
32
in the second joint member
20
. The bore
32
extends at right angles to and intersects the recess
18
. With the sleeve
24
correctly inserted in the bore
32
, the head portion
16
of the fastening element is received in a hollow central region of the tightening element so as to be engageable by the camming surfaces
22
. This essentially is the position of the fastening element, sleeve and tightening element shown in
FIG. 1
a.
In order to form a joint between the two members
14
,
20
the end of the sleeve
24
which is not received in the bore
32
is inserted into the recess
26
in the first joint member
14
. The tightening element is then rotated about its axis of rotation which axis is perpendicular to the longitudinal axis of the fastening element. As the tightening element is so rotated, the action of the camming surfaces
22
is to pull the fastening element axially away from the first joint member
14
. The sleeve is prevented from moving with the fastening element by a sleeve collar
34
which abuts an end face
36
of the second joint member. By virtue of the relative movement of the fastening element and sleeve, the multi-tapered end portion
28
is drawn inwardly of the sleeve causing it to expand and forcibly engage the wall
30
of the recess
26
. Further rotation of the tightening element causes the joint members to be pulled tightly together so that the joint members are in a state of compression and the fastening element is in a state of tension.
A problem can arise with such a joint forming device if the tightening element is not correctly located with respect to the end face
36
of the second joint member. If the tightening element is located too close to that edge it may prove impossible to tighten the joint since only a small portion of the length of each camming surface will be effective on rotation of the tightening element. This results in very little pulling force being applied to the fastening element so that there is little compression in the joint and thus the joint is weak. If the tightening element is located too far away from the edge of the second joint member, unless an undue amount of force is applied, it is only possible to obtain a small amount of rotation of the tightening element in the tightening direction.
The camming surfaces of known camming elements typically have a lengthwise profile adapted to provide a two-phase tightening action.
FIG. 1
b
is a graph illustrating the relationship between rotation of the camming element and axial movement of the fastening element in a conventional joint forming device and showing the two-phase tightening action. Axial movement in mm is indicated along the vertical axis and rotation in degrees is indicated along the horizontal axis. In the first phase, a relatively “coarse movement” of the pin is provided by portions of the camming surfaces having a relatively steep, or tight, curve, i.e. a curvature adapted to provide a relatively large amount of axial movement of the fastening element for a relatively small amount of rotation of the camming element. The relationship between the rotation of the camming element and the axial movement caused is non-linear and is typically in excess of 0.5 mm for each 10° of rotation. This “coarse movement” is provided for closing a gap between the two panels and causing them to be tightly pressed together, a movement often referred to as pull-up. The second phase of pin movement is caused by portions of the camming surfaces having a relatively flatter curve. The second-phase movement increases the compressive force on the joint and the reduced curvature of the camming surfaces imparts a degree of self-locking, often referred to as lock-up, to the joint forming device. It will be appreciated that the steeper the curve of the camming surfaces at the point of engagement with the head portion of the pin, the more likely it is that the camming element will back-off, i.e. rotate in the sense opposite to the sense in which it is rotated in order to tighten the joint when the panels are loaded in use. Accordingly, camming elements designed to provide the above-mentioned two-phase pin movement are intended to be rotated sufficiently during tightening of the joint to ensure engagement between the head portion of the pin and the flatter portions of the camming surfaces to obtain so-called lock-up.
In the use of the camming element which is the subject of
FIG. 1
b,
the camming element is rotated approximately 50 degrees from the position in which the head portion of the pin is inserted into the hollow region before the camming surfaces engage the head portion. The camming element must then be rotated a further 75 degrees before effective locking of the joint is obtained.
There are several disadvantages with these conventional joint forming devices and their camming elements.
One disadvantage is that relatively high turning forces are required to rotate the camming element during the pull-up phase. It will be appreciated that the cam is relatively highly ‘geared’ during the pull-up phase and the steeper the curve of the camming surfaces, the more torque input is required from the user. It is desirable to reduce the effort required of the user.
A further problem associated with this design is that in order to cope with the relatively high forces acting on it, the camming element must be heavily built. If the camming element is not made strong, it may burst under the loads imposed on it; the phenomenon of cam bursting will be familiar to those skilled in the art.
It will be appreciated that such camming elements are produced in vast quantities usually by means of diecasting techniques. Diecasting metals are relatively expensive and it is therefore desirable to reduce the weight of metal required for each camming element as much as possible without significantly reducing the strength thereof.
A further disadvantage of known joint forming devices is that they lack tolerance. That is, if the recesses and bores in which the device parts are fitted are not made accurately, it may not be possible to form a reliable joint.
In more detail, in use the camming element is usually fitted into a recess in a major surface of a first joint member. This recess either breaks an edge of the panel or is communicated with the edge by means of a bore which extends from the recess to the edge. This arrangement allows the head portion of the pin to be inserted into the camming element so that it can be engaged by the camming surfaces on rotation of the camming element.
If the camming element is located too close to the edge of the joint member it may prove impossible to tighten the joint since only a small portion of the length of each c

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