Power Transmission	3-99
	Technical Feature

Chains push performance further

Flexible actuators that push are robust and reliable enough to use even in furnaces. Tom Shelley reports.

Chains which push as well as pull can work in environments much too hot and abrasive for other devices

When retracted, the chains roll or fold up, saving space, yet lock together when pushed out. Limit switches can be incorporated in the driving apparatus.

Typical applications include a quick lifting mechanism for plasma torches in a Japanese steelworks, and conveying systems for ceramic ovens and furnaces. More typical applications are in the automotive sector. One common use is for changing tools on presses. Automotive press tools often weigh one or two tonnes. Other typical uses for the push chains are foundries, nuclear plant, rolling mills and in the chemical industries.

The latest push chains are made by Franz Morat, a 135-year-old family firm based in the Black Forest in Germany, and sold in this country by Drives International. Morat also makes custom designed drives and is a volume manufacturer of worm gear sets. The push chains were originally developed by a French company, which was less than totally successful in making them to the required degree of robustness or precision. Unlike designs in which chains latch together with sprung mechanisms, the Morat product simply has a set of fingers on one side, so is completely flexible when bent in one direction, but totally inflexible when bent in the other.

Previous attempts to produce successful push chains include devices made for domestic windows, and at least two designs featured in past issues of Eureka. Long term reliability in the present design is ensured because push loads are not carried by the pivots and associated bearings, but by the fingers pressing against each other. A special front link prevents the chain from folding when pushed against a load. The front link shifts the point of contact in relation to the joint pin and creates a bending moment in the opposite direction, effectively locking the links into a rigid bar.

Thrusts can be delivered horizontally, at an upwards or downwards angle, or even vertically, provided the far end is constrained, as in a scissor lift. A vertical, unconstrained actuator could, in theory, be made by making two chains unroll back to back. Such a configuration has been demonstrated but not, as far we know, exploited in practice.

For applications requiring extra lateral rigidity, there are double chain versions with three sets of fingers, and for corrosive environments, stainless steel designs. Bearings are bronze self lubricated. Configurations available include several with sprocket driveable side rollers as well as central rollers. One of the big achievements of the current set of designs, is their ability to work in a hot environment, and maintain precision despite differential thermal expansion coefficients. Standard designs can be used at up to 180 deg C or at higher temperatures for short periods of time. For special applications, the company is able to design chains able to withstand temperatures of up to 600 deg C. Samples of chain studied by Eureka showed no detectable play and looked solid and precisely engineered.

Movement control may be achieved by combining the chains with gear motor drive boxes made by the same company. As the chain is driven out or pulled back, a lead screw is made to move a nut within the drive box between two limit switches, also on lead screws. Chain costs are from £400 to £500 for 2m travel including supports for a light weight 25mm pitch chain with 5kN thrust capability. A 2m travel with a 60mm chain and 35kN thrust capability is likely to cost about £4,000. The heaviest chain is a 90mm double width chain with 180kN thrust capacity. Thrust figures are for 1m of unguided chain. Forces can be delivered over much greater distances when the chain is run in a guided channel. Chains can be coupled in parallel to deliver even greater forces. According to the makers, the chains can be used without guide channels when the chain moves across a smooth, flat surface; the total stroke is relatively short; and the chain is not subjected to side loading relative to the axis of thrust.

Prices for the aluminium casting enclosed electric drive units are around £300 for a 0.4kW unit delivering 140Nm or torque or £200 for one delivering 60Nm. The low prices arise because the units are made in large volumes for moving swimming pool covers, among other things.

One surprising use of the chains is as driving elements within a range of telescopic cylinders made by another German company, S & R Automatisierungstechnik based in Homberg. Typical uses for the cylinders are for elevating television cameras, lighting and in stage systems for theatres.

The chain systems can be made to move at higher speeds than leadscrew systems, and do not require space for the withdrawn leadscrew. Withdrawn chains can be accumulated either above or below the driving sprocket. Compared with pneumatic and hydraulic acuators systems, the chains have built-in rotation resistance and required no seals.

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