Tilting elements block all back-driving
Tom Shelley reports on a deceptively simple looking device that could revolutionise a multitude of mechanical drives
An arrangement of tilting elements acting on an indented cam profile can ensure smooth delivery of torque from input to output shaft in either direction of rotation – while firmly locking up when torque is applied in the reverse sense.
Originally invented to stop ship’s rudders being back driven while moored – and damaging steering mechanisms – it has since been applied to industrial doors, where it does away with the need for counterbalances and springs and enhances security.
The technique has also been proven in applications ranging from window blinds to construction equipment. Because it cannot be back-driven under any circumstance, is could put an end to energy-wasting worm drives.
Samar Technology’s anti back drive coupling was originally invented by Raul Montanana at Gloucestershire-based Sam Tooling in 1993.
Andrew Sterry, director of Sam Tooling, says: “We are a sub-contractor, but wanted to iron out the peaks and troughs in our business with a product we could make and sell. So we made a prototype and applied for a patent, finding to our surprise that there was no prior art.”
Sterry thought it unlikely that Samar could come up with a generic product to use in anything, so it identified a few products where back drive was an issue – and sought partner companies.
“Our dilemma was that we realised we could not attract interest without a prototype – but did not want to incur the cost of making one without a clear application,” he says. “So in 2001 we were still in an ‘innovation seeks application’ situation. We then identified industrial doors as a particularly promising application.”
There was a legislative change around this time so the company began to talk about the idea to door manufacturers as a safety enhancement. Most industrial doors have large springs that have to be wound up – and are potentially dangerous to install. With the Samar coupling, there is no need to have a large, mains-powered motor to overcome the friction in the worm and wheel gearbox.
“We can use a low voltage DC motor instead, which does not require a qualified electrician to install it,” he says. “The door maker can offer more features because the DC motor is easier to control.”
The device is now in production, and a DTI grant is allowing Samar to research other potential markets and applications.
Demonstration and production units at Sam Tooling have three or four rotatable elements that are almost circular – but with a cut-out in the form of an arc on their outer sides. In normal driven mode, these are acted on by driving elements between them, attached to a ring. The driving elements act on the rotatable elements in such a way that they do not rotate, but act on indentations in a cam profile attached to the output shaft.
If, on the other hand, torque is applied to the output shaft, the indentations on the cam profile engage with the rotatable elements and rotate them into what is described as a “wedging engagement” with the outer ring.
Sterry explains: “What happens depends on the line of action on the elements. A lot of effort went into choosing the right shapes. It’s all about geometry.”
When Eureka saw some of the Chinese-made components being measured as part of the quality control process, we were told that shapes of the rotatable elements, cam and outer ring sets have to be selected to provide a running clearance of around 5 microns.
One application was a small plastic and metal device that “replaces the wrap spring clutches in roller blinds”. It looked cheap to manufacture yet worked perfectly. It was designed to be assembled robotically.
“We are planning to incorporate it into planetary gearboxes where it would replace the last stage,” says Sterry. “This would allow us to attack the worm gear market.”
He added that it was effective against back driving as a non-reversible worm, but is much more mechanically efficient and does not require a 90 degree change in shaft direction.
Applied to doors, it does away with the need for counterbalance springs and safety devices, and also avoids the need for locks in many instances. The units we saw being manufactured and tested used 36V, 30A motors and an electronic control board. Because the doors cannot be moved by hand if unpowered, they are provided with battery backup and provision to be run via jump leads from a car battery. Applied to the shutters on the backs of truck semi-trailers, they could reduce the number of drivers who damage their shoulders closing shutters, enhance security and avoid the need for drivers to have to get out of the cab in a loading dock in order to unlock or enter codes.
One prototype system was attached to a disabled mobility vehicle, where fitting it does away with the need for a hand brake. A release mechanism allows a carer to push the vehicle in the event of power failure.
Sterry says: “We have divided our development effort into two – low speeds running at six or seven rpm, and higher speeds where there may need to be a mechanism to disengage it. A suitable mechanism to do this has also been developed and patented.”
Other applications being considered include steering mechanisms for off-road vehicles and construction equipment – where back-driving forces caused by driving over rough ground often put a severe strain on drivers. Lifting equipment of all kinds, whether used on construction equipment or stair lifts for the disabled, can be made safe in a simple way with the device, avoiding the need for interlocks and various complicated safety mechanisms. The mechanism could also be applied to prevent back driving of the steering mechanism of wind turbines, and yes – the company is still looking to apply the mechanism to steering mechanisms for boats.
Samar Technology – Sam Tooling
* Simple mechanism prevents the load back driving the input shaft. In normal use, it absorbs almost no energy – unlike a worm drive
* Being flat, and not requiring a change in shaft direction, it is very easy to build into or onto an existing gearbox
* It is being commercially produced for industrial doors, making them simpler and safer to install and use – but has a host of other applications
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