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 Norbar » Quality » Health and Safety » Reaction Plates

Catalogues & Handbooks


Let's Torque Reaction Plates

Torque reaction plates or arms are essential to absorb the reaction forces of powered and manually operated torque multipliers. Getting the right reaction plate for the job is vital, and Norbar Torque Tools can offer a range of torque reaction solutions to suit all applications.

"For every action there is an equal and opposite reaction" is, as every school child should know, Newton's Third Law of Motion. When applied to torque tools it means that when a tool applies a torque to a fastener or other application there will be a (more or less) equal and opposite reaction torque that has to be absorbed.

When using impact or impulse tools the applied torque is absorbed by the inertia of the tool and the operator, but Norbar does not make these types of tool, which can cause the operator health problems if vibration or noise levels are excessive.

Instead Norbar power torque tools and multipliers that rotate continuously with no pulsing or impact, and so rely on a reaction arm or plate to absorb the reaction torque. With some tools applying torques of up to 100,000 N·m the design, construction and use of the reaction plate can be critical.

The standard reaction plate for a Handtorque, which uses a gearbox to multiply the torque applied by the operator by factors of 5, 25 or 125, is a low alloy solid steel bar. This is located by the operator to rest on an adjacent solid structure, such as another bolt on the flange, where it is able to absorb the reaction torque.

Problems can arise however where there is no convenient point for the reaction plate. If for example a fastener being tightened is recessed or in a difficult location the operator may be tempted to bend or extend the reaction plate.

This may cause the plate, tool or socket being used to twist, bend or break, with potentially disastrous consequences for the equipment or operator.

Options include reaction plates that are curved or have sliding feet to make contact with a suitable reaction point, and tubular steel plates that are lighter and more manageable for jobs where torque tools have to be frequently manhandled.

Norbar is also working on an aluminium reaction plate offering greater strength for its weight than steel, but there are drawbacks with aluminium. One of these is that the metal tends to flake  under pressure, as it is where the end of a reaction arm meets the reaction point, and this can produce dangerous splinters. The solution Norbar is working on involves a steel sheath to protect the aluminium at the point of contact.

Where access to hard-to-reach fasteners is needed Norbar can supply nose extensions for hand or  power tools. These also have the advantage of including a bearing where the extension fits onto the tool which absorbs the torque reaction rather than passing it onto the tool.

There are also other factors to take into account when using standard reaction plates. Extending a torque plate beyond its design length by welding on extra material for example may cause the plate to break. Even though the torque remains the same and the force applied to the end of the longer plate is reduced, the bending moment on the plate close to the tool is increased (Fig 1).

Fig 1: Calculation of bending moment for different lengths of reaction plate

Figure 1

If reaction is taken at distance L1 then the reaction force, F1 can be calculated. This generates a bending moment of (F1 x a) at section xx. If the reaction plate is extended and reaction is taken at L2, then the reaction force is reduced and becomes F2. In this case the bending moment becomes (F2 x b) which will always be greater than (F1 x a), therefore the bending stress at section xx will be greater. In the second instance the reaction plate may fail.

Where flat reaction plates with radiused ends are used to react with a surface that is not perpendicular to the axis of the reaction plate itself the force applied to the end of plate may increase dramatically (Fig 2).

Fig 2: Calculation of reaction force on radiused reaction plate

 Figure 2

In the case of the curved end of a flat reaction plate the line of action of the reaction force passes through the centre of the curve. The further round the curve that reaction is taken the smaller will be the perpendicular distance, L, and in consequence the greater the reaction force, F.

Torque Reaction tools are a safe and ergonomic solution to any high torque turning requirement. Following the guidelines above will improve tool life and ensure their safe and efficient use.

 Let's Torque Reaction Plates PDF version


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