M ARYLAND METRI C S
P.O.Box 261
Owings Mills, MD 21117 USA
(410)358-3130 (800)638-1830 Faxes: (410)358-3142 (800)872-9329
http://mdmetric.com
techinfo@mdmetric.com
TECHNICAL INFORMATION and DATA
Locking of fasteners
When bolted joints are put under dynamic stress, they may become loose unless they are properly secured.
A distinction is made between two types of fastener locking.
a) Securing against loss of clamping load (setting)
With axially stressed, preloaded screws, setting occurs (i.e. smoothing (= local plastic deformation) of the surfaces in the
parting planes, compensation of excessive surface pressure) and leads to a loss of clamping load. A drop in preload
increases the dynamic portion of the load in the screw and can cause fatigue failure.
b) Securing against rotation (unscrewing/loosening)
If a relative movement between the tightened parts is able to occur as a result of high lateral force (or insufficient preload
due to loss of clamping load, faulty assembly, or incorrect dimensioning), it releases the self-locking in the thread and the
friction in the head or nut section and the fastener will loosen by itself. This can, in fact, result in a total loosening (falling
apart) of the bolted joint.
Precaution against loss of clamping load
Effects
Setting partially compensated by tightening
Setting partially compensated during tightening
Increase clamping/diameter ratio, length = 4 x diameter
Greater elasticity, more space for compensation of
setting
Reduce surface roughness, clean separating seams,
minimize the number of parting planes
Less possibilities for loss through setting (embedding)
Use flange products
Larger bearing surface prevents exceeding the max imum permissible surface pressure
Use spring elements with sufficient rigidity
Compensation of setting (too soft spring elements have
no effect, rather they merely increase the number of
separating seams and setting possibilities!)
Re-tighten after initial operating time
Compensates setting
Precaution against unscrewing
Effects
Tighten properly, increase preload, use larger
dimensions, higher property class
Increased axial forces prevent lateral movements
Increase clamping/diameter ratio
(length = 4 x diameter)
Easier bending of the shank. Prevention of relative
movement below head or nut
Use shoulder bolts, pin parts together
Prevention of lateral movement (slip) between parts
Increase grip on head and nut surface
Helps prevent relative movement under head or nut
Increase friction in thread
Unscrewing torque increased
Precaution against loosening / (falling apart)
Effects
Secure against loss of clamping load and unscrewing
No basis for loosening
Increase friction in thread
Even when clamping force is fully lost, a minimum
torque still exists and prevents a complete unscrewing
Limit further rotation through shape locking
T 78
Nut cannot be turned beyond the stop.
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