Posted at 21-Sep-2018

The Mechanical Fuse for Extruders

Paul Skade
By Gareth Lenton
General Manager, Power Transmission

I joined WYKO in January of 2000, the dawn of a new millennium and a completely new start for me after spending 2 years backpacking through South East Asia and Au...

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What’s the difference between single screw and twin screw extruders? Twin screw extruders have twice as much chance of screwing up. All it takes is over-feeding or foreign body ingress and at best you have an extruder jam on your hands. At worst, you have damaged screws or a damaged gearbox. Unless, that is, you’ve fitted the Rexnord XG Series Autogard Torque Limiter.

Plastic Bottling Line

 

The Autogard Torque Limiter works like a fuse in an electrical circuit, which prevents a current overload from spreading throughout the circuit and affecting everything that’s connected to it. In the same way, the Rexnord Autogard Torque Limiter prevents the effects of an extruder screw jam from damaging the gearbox. 

Fitted between the motor and gearbox, the Torque Limiter detects a jam as soon as it happens, and instantly disengages the driving motor from the extruder gearbox: disconnecting and separating the inertias. 

The Rexnord XG Series also has a feature which sets it apart as the best on the market. 

As a completely free-running torque limiter, when the Rexnord XG is disengaged it will continue to run even with the motor still spinning. There’s no need to turn the motor off, and even if the jam and disconnection aren’t spotted immediately, there’s no additional harm done. This free-running capability also means there’s no wear, and less risk of damage, to the Autogard itself – which means it has a longer service life. 

But that’s not all that’s better about the XG Series.

Best of both worlds

Rexnord Wrapflex is an elastomeric coupling which allows for misalignment of the gearbox and screws, but doesn’t offer torque limiting. Now the new Rexnord XG Series combines the misalignment safeguards of Wrapflex with torque limiting protection, to provide the best of both worlds. 

In addition, the XG Series is quick and easy to reset, with no special tools required. 

The distinctive features and functions of the XG Series are a result of extensive consultation with global OEMS, to understand exactly what’s required from a torque limiter. By analysing shaft diameters on motors and gearboxes, Rexnord have been able to design the XG Series to the smallest possible size at the lowest possible weight, to help reduce cost and wear while still providing optimum protection for your capital equipment.

Successful drop-out

When disengagement of driving and driven sides is needed for repair or maintenance, the Rexnord XG Series makes it a quick and easy manual process. And the same process in reverse makes it just as quick and easy to set-up. 

Should you ever need to replace the limiter, a “replace in place” drop-out option allows you to remove the limiter from its position between the gearbox and motor, without having to move either of them. Since they are often bolted in position, that’s a time-saving option which can greatly reduce downtime. 

Whether it’s a result of maintenance, repair, or an extruder jam, downtime can cost a business many thousands of pounds in lost production and financial penalties from customers. So the effectiveness of the Rexnord XG Series Autogard Torque Limiters means they’re not only a mechanical fuse for your extruders. They can also stop your Production Manager from blowing a fuse. 

 

 

Rexnord in Action: Dropping the Clutch

A customer’s plastic extruder drive, used for extruding plastics for the production of bottles, operates under extreme conditions at the limits of its performance. This makes it essential to have an accurate and reliable overload device to protect the equipment from damage. 

The extruder’s gearbox was originally supplied with a friction clutch for overload protection. However this proved unreliable, and required frequent maintenance due to wear. Adjustment was also inaccurate, resulting in costly production stops. 

 

Rexnord Autoguard

An Autogard XG Series Torque Limiter was retrofitted to a new gearbox, and accurately adjusted to protect the extruder from torque overloads, to within 5% of the setting torque. The ball detent design of the Autogard XG also helped to reduce operational wear. 

By providing accuracy, repeatability and reliability, the Rexnord solution resulted in an overall improvement in production, as well as protecting the customer’s assets and reducing their total cost of ownership. 

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