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Correct Fitting of Over-Centre Fasteners

Before using an over-centre fastener or toggle latch on your product, it is important to appreciate how such a fastener works. Our experience has often shown faulty fittings and unsatisfactory performance originating in the design stage of the product. The consideration of a few important points before finalising your product design can prevent having to pack out or bend a toggle fastener at a later stage to make it function properly. Consider:- 

1. How much pull or pressure is required?
2. How much throw or clearance between the catch and the claw of the latch in the open position is required?
3. What are the relative positions of the mounting surface and the point where the claw engages?
4. Is corrosion a problem?
5. Is vibration a problem?
6. Can points of attachment, particularly on castings and mouldings be incorporated in the design and thus eliminate riveting and welding?

 We draw your attention particularly to point 3. The correct locking position (i.e. when the toggle latch has gone over the centre and locked fully home) is an angle between 12º and 15º over centre. If this angle is too great then much of the pressure is lost. However if this angle is much less there is certainly more pull but the catch is not securely locked.

Please note with point 5 that is the product is subject to vibration, consideration should be given to the use of a flex link rather than a straight link or claw. This would give a more secure locking under those circumstances.

For further technical advice on these matters please contact us.

In a normal situation where the point of attachment (A) is in line with the base of the toggle latch (see figure 1) all standard toggle latches can be used. The locking angle (x) is approximately 15º

In a situation where the point of attachment (A) is well past the mounting surface (see figure 2) some pressure may be lost through the excessive over centre movement. If a straight wire link is used the connection may be slack. To minimise these possibilities the attachment hole (C) can be pierced on the centre line of the pivoting hole (B) and thus decrease the over centre movement.

In a situation where the point of attachment (A) is above the mounting surface (see figure 3) the two angle toggle latches (S3-B3A/L2 or S3-B1A/L2) could be used. These toggle latches can be formed at their base to the correct angle to reach the point of attachment (A) thus maintaining the correct locking angle.