Electricity + Control

By Eldon Kruger, Pratley

Most South Africans are aware that Pratley Putty is the only South African manufactured product on the moon.

George Montague Pratley should however be remembered more for the brilliant engineering he applied to his patented ‘Adjustable Cable Gland’.

Never regard a cable gland as just a lowly nut at the end of a piece of cable. A cable installation risk analysis puts things in perspective. Assuming the cable is properly sized, made by a reputable manufacturer and intelligently routed, it is apparent that the real risk lies at its ends where it's terminated. The risks include, power failure, fire, corrosion, water ingress, electrocution and the most common one; the financial risk of having to redo, the installation.

A cable gland with a virtual threadThe Pratley family: Top: Kim Pratley – son of George Montague Pratley and CEO of Pratley; Kim’s wife, Val; their sons Charles, and Andrew.

The quality of a cable installation is also materially influenced by the quality of the gland. If it's easy to fit, it's generally installed more professionally. So what's so special about Monty Pratley's Adjustable Cable Gland? Well, lots really, but two things in particular.

Firstly, it's adjustable. By simply turning the nipple by hand, the gland can be adjusted to grip absolutely any diameter of cable armour wire. This is unique and it means that unlike the case with ordinary glands typically with fixed or captive cones, the electrician will never find that he is unable to mate the gland threads due to oversized armouring.

Similarly, he will never tighten the gland only to find that the armouring or braid has not been gripped because it is too thin for the gland. Although it's seldom necessary, this adjustability also means that large adjustable glands have the ability to terminate small cables as well.

Secondly, the gland has a virtual thread. A cable gland's primary function is to grip the cable (its armouring).

Other things being equal, the better the grip, the better the gland.

So, the Adjustable Cable Gland is already adjusted for the exact size of armouring as described. Now comes the really clever bit, a virtual thread! It's well known that for any given torque, the clamping force exerted by a thread is greater if it's a fine pitch thread. Problem – fine pitch threads are expensive to make, easily cross threaded, easily damaged and difficult to engage. Solution – a gland body; or 'differential nut' in this case, with two coarse threads each of slightly different pitch. So, how does this work?

When the gland is tightened, it does so according to the pitch of the coarser of these threads. However, whilst doing so, it simultaneously loosens according to the slightly finer pitch on the other thread. The nett result is a virtual thread whose pitch is the difference between these two pitches. This virtual thread can be made infinitely fine.

The upshot is a gland with two relatively coarse pitch threads but which achieves a very fi ne pitch thread action without suffering any of the fine thread drawbacks.

This results in an incredible cable armour clamping force even with a very modest tightening torque. Indeed, an adjustable cable gland can often outperform specification slip load tests when just hand tightened. It's like having a built-in gearbox.

The adjustability and differential thread action are made possible by incorporating a loose cone bush. This has many other technical advantages as well. It can self-centre under the wire armours to achieve the best average grip position and it renders the armour clamping both visually verifiable and independent of axial movement of the cable during tightening. This latter effect is a common cause of cable gland pull out failure from ordinary glands. Price is different to cost and when considering total cost, the integrity, reliability and longevity of installations are important parameters. So too is the often ignored time cost of the labour required to install specifi c items. Large projects may involve thousands of cable glands. If it takes just five minutes longer to fit an ordinary cable gland, that's 10 extra artisan days per thousand glands fitted.

When it comes to pure price, it's useful to express the difference in price between various cable glands in terms of an equivalent length of cable. For a typical PVC 4 mm2 x 4 core armoured cable the cable length equivalent of the price difference between a top quality adjustable gland and the very cheapest eastern import is a mere 9 cm of cable. Coincidentally, this is also the equivalent cost of about one minute of artisan time. If on a particular project the average cable run was say 25 m, the price saving would be just a fraction of a percentage of the cable price. The cost saving on the other hand would be negative.

Conclusion

Given all of the technical advantages, installation cost savings and lifetime cost savings and of course the extra margin of safety, the use of Adjustable Cable Glands for terminating armoured cables makes good sense. Complexity can solve most engineering problems; simplicity however requires more skill. The design brilliance of Monty Pratley's Adjustable Cable Gland lies not in complexity but in its elegant simplicity.

 
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