Mechanical Technology July 2016

Mechanical Technology — July 2016

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Power transmission, bearings, bushes and seals

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“

MG’s portfolio of over

15 000 locally manu-

factured and imported

fastener stock items are

available in a variety of grades, materials

and strengths that meet stringent quality

and safety specifications,” says Andrew

Johns, manager, BMG’s fasteners, tools

& equipment. “These fasteners are

enhanced by products that prevent the

loss of joint clamp. Equally important are

installation and disassembly tools that

ensure each fastener can be correctly

installed and efficiently removed.

“For a fastener to be effective, key

factors that require attention are the

type of bolted joint design required and

whether the joint should be serviceable

or permanent. There are two types of

joint for consideration – tension joints or

shear joints.

Effective tools and fasteners

enhance reliability

BMG’s Nord-Lock multifunctional wedge-locking solution combines a wedge-effect to prevent spontaneous

bolt loosening with a spring effect to compensate for loss of preload due to slackening.

BMG’s Superbolt multi-jackbolt tensioners,

which form part of the Nord-Lock range, are

designed to reduce downtime and eliminate

unsafe and laborious bolting methods for

equipment, particularly in harsh operating

conditions. They are shown here being used

to secure the guides of a forging press.

BMG’s extensive range of fasteners, tools and related equipment is used to

enhance reliability of equipment in diverse sectors, including light, medium

and heavy industries, pulp and paper, construction, original equipment

manufacturing and agriculture.

“Tension joints are simply explained

by the tension created on the bolt and

clamped components of the joint, by

stretching the fastener when torque is

applied during tightening. This torque

needs to be accurately calculated to

ensure that the fastener’s limits are not

exceeded. Conversely, the application of

insufficient torque and thus an ineffective

clamp of components, could mean the

joint will self-loosen prematurely.

“The second type of bolted joint uses

the shear strength of the bolt to transmit

the load and often relies on a locking

mechanism to retain the joint integrity as

opposed to a high clamp force.”

If a serviceable joint is required, a

number of factors need to be considered

to ensure that bolt-securing elements

are effective and that the joint tension

is maintained over the life of the joint.

These factors include thread size, clamp

length ratio, preload, amplitude, lubrica-

tion and surface quality.

The selection of appropriate assem-

bly and loosening tools, coupled with

accessibility to the joints, is critical to

overcome factors that cause the loss of

clamp and to ensure ease of assembly

and disassembly.

Some of these products include

Nord-Lock wedge locking washers,

which when tested according to DIN

25201-4:2010-01, have a remaining

joint clamp load of more than 80%

after 2 000 cycles. Nord-Lock washers

are available in standard, over-size and

X-Series for softer materials or flexible

joints, as well as stainless steel to combat

corrosion.

BMG’s Nord-Lock bolt securing sys-

tem uses tension instead of friction to

ensure that joints holding sub-assemblies

together remain secure. Bolts need to be

easily removable during maintenance

procedures, but must resist the loosening

effects of vibrations and dynamic loads.

Bolts that are locked conventionally by

friction between components tend to lose

most of their preload through vibration,

while those locked by the tension of this

system present only minor preload loss,

which is caused mainly by settlement

between contact surfaces. Even after

moderate tightening, the bolt is safely

locked when using Nord-Lock’s wedge-

locking method.

Nord-Lock washers, which are in-

stalled in pairs, cam face to cam face,

have cams between them with a greater

rise than the pitch of the bolt thread.

When the bolt and/or nut is tightened,

the radial teeth on the opposite side grip

and seat the mating surfaces. The teeth

also lock the washers in place, allowing

movement only across the face of the

cams. If the bolt or nut starts to rotate

loose in relation to the other, the resulting

tension caused by a rise over the cams

Nord-Lock X-series

washers.