Large enclosures: it's an open and shut case
25 June 2013
Large cases and enclosures frequently have heavy lids and doors. From a user point of view this becomes an issue when the supported weight is too heavy for comfort, the weight is lighter but the frequency of opening is high or the application requires a lid to remain open without support.
In these situations gas struts are a better alternative to more traditional locking hinges and also far cheaper than actuators or spring loaded return arms.
But hitherto, there has been a bit of a problem with gas struts - their single fixed load rating. An application, under varying operational conditions, will require different degrees of load to be taken by the gas strut. This can make specification difficult in terms of selecting a suitable pressure, especially for single, variable or short-run applications.
Manufacturers will charge struts to a specified gas pressure, but part costs or order numbers typically end up being rather high relative to the basic product value. Fortunately, however, adjustable gas struts are now available, which allow users to tailor the product to their exact needs.
Large metal or composite enclosures often have large and heavy doors or other forms of closure and these can be either awkward or difficult to lift and subsequently maintain in the open position.
Pioneered by the automotive industry for boot and bonnet retention, a pressurised gas strut is frequently deployed to share at least some of the required opening force and reduce the apparent weight of these heavy steel lids for the user.
Depending on the design intent, this may extend to supporting the entire weight of the lid or, where self-opening boots and bonnets are concerned, applying slightly more force to effect an opening movement.
Because the range of possible uses is so large, and as its available thrust being out by just a few Newtons can render a gas strut unfit for purpose, many suppliers are now supplying self-adjusting gas struts the forces of which can be easily modified to suit individual requirements. Chris Putman of the UK based component supplier WDS, takes up the story:
"If you were to take a sample group of a hundred applications where a gas strut was required to support an enclosure lid I would happily bet that no two applications would require the same force. Some applications require the gas strut to simply reduce the weight of the lid while another may require the lid to be held in place. In the first example the accuracy of the force isn't as critical so long as it does not exceed the closing force of the lid; however the second requires the exact closing force of the lid to be matched.
"Because there is such a wide range of pressure requirements it isn't possible to offer a complete range of pre-calibrated gas struts as standard without incurring extended lead times or the need for large warehousing facilities. Stocking adjustable gas struts means that unit prices are low, and delivery times are short – typically next day from us, while product flexibility remains high."
Most adjustable gas struts on the market are extremely easy to calibrate, requiring simply an Allen key or screw driver to open a valve which releases pressure from the chamber. Once the first gas strut has been correctly calibrated it is easy to calculate the required force for the application by applying increasing loads to the gas strut until it begins to depress.
It should be noted that, once released, pressure cannot be added so it is important to release the pressure slowly on a test sample. Chris Putman again:
"We have developed our product to be as user friendly as possible so you certainly don't have to be an engineer to use them. Firstly we supply an adjustment key with every product, meaning that no additional tools are required for calibration; secondly we can supply a simple equation to help users work out the required force for their application.
"Where an OEM may require many gas struts to perform the same function then, once they have identified the ideal pressure for their application, we are able to supply them with fixed pressure products which have be pre-loaded with the correct force. The products in our range are able to supply forces of between 20N and 2,500N."
WDS supplies a range of standard parts for machine builders, including work-holding jigs and fixtures. Some 15,000 parts are currently held in stock and the company says it will not impose surcharges for small order placements. There are five main divisions: Standard Parts, Machine Accessories, Spencer Franklin Hydraulic Workholding, Broaching and General & Modular Work-holding.
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