Advantages of Welded Body Hydraulic Cylinders

There are primarily two styles of hydraulic cylinder construction used in industry, tie rod style and welded body style. ( A third design style, the crimped barrel design, is used in small bore, low pressure applications and dominates the industrial non-repairable, low cost market. A fourth cylinder design style is the tie tube design. It uses retaining rings inserted into grooves on the inside diameter of the barrel and the O.D. of the heads. This style is also limited to low pressures and relatively light duty.)

The tie rod style of cylinder uses high strength threaded steel rods to hold together the two end caps and the cylinder barrel. This method is most often seen in factories and is also a common design for air cylinders. This style of cylinder can be completely disassembled for service and repair.

Welded body cylinders have no tie rods. The barrel is welded to the end caps and the ports are welded to the barrel. The front rod gland is usually threaded into or bolted to the barrel thus allowing the piston rod assembly and the rod seals to be removed for service.

Welded style cylinders dominate the mobile hydraulic equipment market and the heavy equipment industry. This is because of the distinct advantages that the welded body design has over the tie rod design.

There are four distinct features that make welded cylinders the style of choice in many applications.

Tie rod cylinders usually have large square end caps which are much larger than the bore size of the cylinder. These are often a challenge to fit in many confined areas of machinery.

Welded body cylinders have an overall diameter just slightly larger than the effective bore size of the barrel. Basically they are as wide as the outside diameter of the barrel. The only other protuberances are the port fittings and mountings. The net effect is a slim actuator that more easily fits into the tight confines of machinery.

The welded body design is more space efficient allowing a larger bore, more powerful cylinder to fit is the same space as a smaller bore, but bulkier, tie rod cylinder.

Welded body cylinders are also space efficient in their overall length compared to tie rod cylinders. Their mountings are usually welded to the cylinder ends providing compact but strong mounts.

Tie rod cylinders can be built tough and some designs are available for service pressures up to 6000 psi. The strength of the design is determined by the strength and diameter of the tie rods and their fasteners. Tie rod cylinders up to 8" bore usually have 4 tie rods joining the square end caps. These tie rods often have a 100,000 psi yield strength to contain the forces being generated by the actuator. Larger bore, high pressure tie rod cylinders may have as many as 24 tie rods joining the end caps.

Long stroke tie rod cylinders face several problems inherent in the design:

Any steel component under tension stretches to a certain degree. The longer the piece of steel in tension, the more the total stretch. This becomes a problem for a long stroke high pressure tie rod cylinder. Whereas the barrel is not under tension, the long steel tie rods may stretch to such an extent that the barrel separates from the caps resulting in a loss of pressure and fluid. Tie rod cylinder manufacturers often specify in their catalogues the maximum allowable cylinder stroke at various pressures before tie rod stretch will cause component failure. On small bore (1-1/8") hydraulic tie rod cylinders the maximum stroke may be as short as 30". One manufacturer of tie rod cylinders, publishes a maximum stroke of 170" for all bore sizes larger than 3" due to the limitations of the basic tie rod design.

A long stroke tie rod cylinder will often be fitted with one or more intermediate heads along the barrel to prevent the long skinny tie rods from sagging due to their unsupported weight. Tie rod cylinder manufacturers often specify in their catalogues the maximum allowable cylinder stroke before intermediate heads must be installed along the barrel to prevent excessive tie rod sag.

Tie rod cylinders are also susceptible to twisting and flexing. The end caps are free floating and bonded only by the tie rods. Many tie rod cylinder manufacturers will publish pressure limits and mounting style restrictions because of flex and twist issues inherent in the design.

Welded body hydraulic cylinders are not susceptible to the problems tie rod sag or stretch. This is because the barrel of a welded body cylinder is a fully stressed supporting member of its construction. This contributes to the overall strength of the design. The barrel is not just a pressure vessel but also holds the end caps together. The barrel will stretch under tension but this will not cause leaks as the heads are bonded completely to the barrel.

Long stroke welded body cylinders use the entire diameter of the barrel as a supporting member to resist sag. The barrel is welded along its entire outside diameter to the head thus providing an incredibly strong bond. This also prevents twisting and flexing.

The overall design of the welded body cylinder lends itself to customizing and special configurations. Ports can be located easily anywhere and at any angle. Mountings can be welded anywhere along the whole length of the actuator.

This inherent capacity of flexible design makes the welded body cylinder ideal for OEMs and custom machine builders. An actuator can be fit into almost any application with greater ease and less cost than customizing an equivalent tie rod cylinder.

Welded body cylinders have a smooth exterior surface free of tie rods and fasteners. This means that there are alot fewer places for dirt and debris to cling to. Accumulating dirt, over time, can become a corrosion problem that will lead to failure. The round body on a welded cylinder is free of obstructions allowing it to be easily cleaned.

That smooth uncluttered exterior is also free of places for things to snag onto. There are no tie rods and no big square pointy heads to catch onto wires, hoses, body parts, brush, and so on.

Because the welded body design is used extensively in many low cost markets and built in low grade versions by many 3rd world manufacturers using inferior materials, many designers have come to view the welded body hydraulic cylinder as a cheap and dirty type of cylinder. This view is quite mistaken.

As shown above, welded body cylinders have some distinct advantages in a number of key areas over tie rod style cylinders. When designed and built by a high quality manufacturer using high grade materials, the welded body hydraulic cylinder provides rugged and powerful solutions to many actuator applications. It is no mystery then why this design dominates many key hydraulic actuator markets around the world and is used in many very demanding applications.