WINTER 2024

104
THE DISTRIBUTOR’S LINK
BRUNO MARBACHER SECURING BOLTED JOINTS from page 30
Screws and nuts have only 2 areas where friction is
effective:
¤ Underneath the head and/or the bearing
area of a nut
¤ In the threads
The 3 Common Ways Bolted Joints
Become Loose
Straight-loosening (slacking) of bolted joints -
When the clamped parts have a too low surface hardness,
the joint may relax excessively or the nut and/or screw
head may embed into the clamped parts. Both scenarios
are causing bolt loosening. Rough surfaces of joint
members, thick coatings, or thick hot dip galvanized layers
may also lead to joint loosening.
To Prevent Clamp Load Loss
Surface roughness must be reduced or locking
elements such as Belleville washers, split lock washers
or wavy washers must be utilized to compensate for clamp
load loss.
Because of their spring effect, they can compensate for
a possible relaxation of the bolted joint or a drop in clamp
load. This does only work with low strength fasteners such
as machine screws etc. The element’s spring load must
be higher than the clamp load.
A further option is to use Joint member materials
with higher surface hardness. If that is not an option,
a large diameter hard washer ought to be used. In any
event, testing and/or calculating whether the diameter or
hardness in the bearing area is sufficient to deal with the
induced surface pressure is necessary.
The ideal option is using flange bolts and or flange
nuts, they have a larger bearing area. Thus, reducing the
surface pressure on the clamped parts. This results in
less preload loss; a loosening of the joint occurs less
often or not at all.
Rotational (spontaneous) loosening of screws
or nuts - Fasteners rotate loose (turn), when clamped
parts vibrate. Dynamic shear forces act on the bolted joint
causing the joint members to slip back and forth. This will
prompt screws and nuts to rotate, reducing the preload to
zero.
Loss of screws or nuts, coming apart - Under
constant dynamic forces the screws/nuts slack off until
they fall apart. Generally, rotational loosening precedes
this action.
To Prevent Rotational Loosening
A simple way to maintain the self-locking effect is to
increase the clamping range to about 5 times the screw’s
diameter. Because the bolt shank is longer the bolt head
and/or nut can follow the sliding of the joint members
without causing wobbling and sliding in the critical areas.
There are of course cases where increasing the clamping
range is not possible. That being the case, there are two
basic ways to prevent rotational loosening.
Ways To Address Loosening
Once one has determined the potential cause for
loosening one can go about finding a locking element
that prevents it. Choosing the right element is crucial so
let us look at the individual situation and find the proper
element.
CONTINUED ON PAGE 146