Rod – Rod diameter is a key consideration as it effects maximum force. The most common rod outer diameters for gas springs are 6mm, 8mm, 10mm and 14mm.
A rod comes as either precision ground carbon or stainless steel and the surface is treated to improve wear and corrosion resistance. Carbon Steel can be treated in several ways such as chrome-plating, salt-bath nitriding or a gaseous nitriding surface layer treatment. For carbon steel rods, gaseous nitride surfacing has several advantages over the former methods. These include:
- Better wear resistance
- Lower frictional characteristics
- Increased corrosion resistance
- Non-toxic and produces no acidic by-products
Tube – As with rods there are common sizes for outer tube diameters. These are 15mm, 18mm, 23mm and 28mm. Gas spring tube material is high-integrity, carbon or stainless steel seamlessly welded; which is suitable for high pressures. The internal surface finish and tensile strength of the tube are critical for a gas springs longevity and burst pressure performance.
Rod and tube combinations – It is possible to mix and match the rod and tube combinations for specific application and performance purposes, however the traditional and most cost-effective size combinations are: 6-15, 8-18, 10-23 and 14-28 e.g. 6-15 is a Ø6mm rod and Ø15mm tube.
To give an indication of just how precise the tolerancing is on quality gas springs a tube surface roughness of less than 0.5μm Ra and roundness 0.05mm should be expected.
Stroke – The stroke is the maximum distance the rod can travel from being uncompressed to fully compressed. The rod will always be longer than the stroke needed by the application and shorter than the length of the tube.
Extended length – The extended length is the total uncompressed length of the gas spring measured centre to centre between fixing methods (end fittings).
Closed length – The Closed length is the total compressed length measured centre to centre between the end fittings.
There may be times no end fittings are specified. If this is the case, the measurements should be taken from rod end to tube end excluding threads.