Welded Style Hydraulic Cylinders vs Other Designs

Welded body cylinders are used widely in industry in the most demanding applications. They are manufactured in bore sizes from 1/2 inch up to 30 inches and even more. In fact, welded body cylinders are the design of choice for heavy duty industrial and mobile equipment applications. The dominance of the welded cylinder style in tough applications is due to the inherent strength of the design when compared to other methods of building cylinders.

The Different Methods of Constructing Hydraulic Cylinders

There are many different ways to build hydraulic cylinders but not all of them are suitable for retaining high pressures, being used for long stoke lengths, or enduring the loads seen in heavy duty applications.

Crimped Barrel Cylinders

One common method of building cylinders uses a thin walled, stainless steel tube that is crimped onto aluminum end caps. This method of assembly is considered non-repairable or "throw-away" as the barrel crimp is permanent and leaves no easy way to access piston and cushion seals for replacement. Some designs enable replacement of rod seals and bearings, but most do not. This style is generally used for air cylinders in small bore sizes up to 3 inch diameter and is limited to hydraulic pressures under 500 psi. This is generaly considered to be a light duty design.

Threaded Barrel Cylinders

Another method of cylinder assembly uses an aluminum barrel that is attached to the heads by threading the inside diameter of the tube onto the end caps. This method enables disassembly for repair but is relatively weak and often results in poor alignment between the heads and the tube. Sizes are usually limited to about 3" bore and pressures to a maximum of 500 psi. This too is generaly considered to be a light duty design.

Retaining Ring Cylinders

A similar method uses steel retaining rings that slide into matching grooves on the inside diameter of the barrel ends and the outside diameter of the end caps. Again, it enables disassembly for repair but the connection between the heads and the barrel is relatively weak. Pressures are usually limited to about 500 psi or less. Sizes are limited to up to 4 inch in standard rod cylinders and 8 inch in low profile or "pancake" cylinders. The retaining ring style is also generaly considered to be a light duty cylinder design.

Tapped Barrel Cylinders

Another method of constructing cylinders uses a heavy walled barrel of steel or aluminum. The thick cylinder wall is tapped on the ends to accept small diameter screws. The end caps are then attached to the barrel with a multitude of fasteners in a ring around the barrel. This method is usually used only on low profile or "pancake" cylinder designs as it enables the attachment of very short end caps. This style is fully repairable and can accept higher pressures depending of the strength of the fastener connection. It is much more costly, however, because of the thickness required for the barrel material to be tapped. It is considered to be a medium duty design for low profile cylinders and can generally accomodate pressures up to 2000 psi.

Tie Rod Cylinders

The tie rod design of building cylinders is very common and can be found on cylinders from 1/2 inch bore up to 24 inch bore. This method uses high strength steel tie rods to join the opposing cylinders ends caps with the barrel sandwiched in between. Very small bore cylinders may only use two tie rods. Most, though, start by using four tie rods passing through the corners of the square headed end caps. Large bore tie rod cylinders, usually from 10 inch bore size and up, require more than 4 tie rods in order to retain the large forces being produced by the pressure within the cylinder. Very large bore tie rod cylinders may have as many as 20 tie rods.

Tie rod cylinders are easy to assemble and disassemble. They are, thus, fully field repairable including easy replacement of all the seals, bearings, and even the barrel. Repair can be done very quickly and often with very basic tools. Manufacturers like the tie rod design because they are generally simple to build, even custom stroke lengths. The National Fluid Power Association (NFPA) has developed standard specifications for tie rod cylinders so that many manufacturers build them to external dimensionally interchangeable mountings. This allows end users to easily choose between different suppliers for competitive cylinder sourcing.

The tie rod style of cylinder is available in designs from light duty up to heavy duty. Pressures up to 5000 psi can be accomodated.

The tie rod cylinder design does, however, suffer from some short comings. These are related to the use of tie rods to retain the heads. These disadvantages become more and more evident in larger bore sizes, longer strokes and higher pressures.

The strength of the tie rod cylinder design is, of course, limited to the strength of the relatively thin steel tie rods used to retain the ends caps. At high pressures the tie rods stretch. If the tie rods stretch too much, the barrel will actually detach from the end cap and hydraulic fluid and pressure will be lost. This would be a catastrophic failure.

The thin steel tie rods are also subject to sag. Tie rod sag becomes very evident on long stroke tie rod cylinders. If left untreated, a long stroke cylinder would fail upon being pressurized. The pressure would cause the end caps to push apart and the sagging tie rods would straighten under the opposing forces. The distance between the end caps would increase and the barrel would float freely in between. Structural integrity would be lost as well as the fluid pressure when the barrel departed the end caps. For this reason, tie rod cylinders with very long strokes are often equipped with intermediate heads to support the weight of the thin rods. This adds to the complexity and cost of the actuator.

The relative weakness of the tie rods can also become apparent when a cylinder heads come under severe torsional or off-center loads. A cylinder may come under a twisting load and the opposing heads with deflect. This may result in a permanent structural deformation or it may cause the piston to bind within the cylinder. Likewise, an off center load on the heads, such as is experienced by a foot mounted cylinder, may cause the heads to deflect on an angle. This will apply side forces to the rod and cause premature wear. Tie rod cylinder manufacturers often publish pressure limitations for their cylinders using side lug, side tapped, and foot mounts.

Welded Body Cylinders

Welded body cylinders do not suffer from the structural weaknesses of these other cylinder designs. In fact, welded cylinders bring to the table some additional advantages that help greatly in machine design.

In a welded body cylinder the cylinder barrel is a fully stressed member of the overall structure of the actuator. The rear head is welded to the barrel. In small bore sizes, the front head may be threaded to the inside diameter of the barrel. In larger bore sizes, the front head is bolted to a flange that is welded to the barrel. The barrel is a thick walled steel tube suitable for welding and use as a structure.

With this design, there is no problem caused by metal stretching at high pressures or sagging on very long strokes.

Torsional and off center loads do not cause the problem of deformation or flexing as in tie rod cylinders. The strong barrel to head connections, welds or bolted flanges, do not allow these to occur.

The absence of external tie rods also make welded cylinders small in outside diameter compared to their tie rod cousins. Thus a machine can often be made smaller or a larger cylinder can be fitted in the same space by using a welded body cylinder.

The lack of tie rods and the use of a thick walled tube also enables the machine designer to customize a welded cylinder much more readily than other cylinder designs. Mounting attachments, for instance, can be added virtually anywhere along the body of a welded cylinder.

The smooth round outside surface of a welded cylinder is yet another advantage. Unlike other designs, there are fewer sharp corners jutting out to snag hoses, wires, other machine parts or personnel. The smooth uncluttered outside surface provides few places for dirt and moisture to gather and corrosion to set in. A welded cylinder is thus also easy to prepare, prime and paint with a consistent even coat.

Although the rear head is usually permanently welded to the barrel, the front head is usually removable for servicing the rod seals, the rod bearing, the piston seals, piston bearing, the piston rod, and inspecting the inside finish of the barrel. The barrel of a welded cylinder is usually thick enough that if it has been damaged inside, it can be honed or burnished several times without affecting the structural integrity of the whole unit. The barrel is usually thick enough that it is resistant to much external damage as well including nicks and dents that would ruin the thinner walled tubes of other cylinder designs.

Because of their inherent strength, welded body cylinders are used in all ranges of industrial hydraulic pressures up to 5000 psi and beyond. Their great structural strength and integrity makes them the best design for very heavy duty and demanding applications. For these reasons, welded body cylinders are used extensively in heavy industry, mobile construction equipment, miltary hardware, and in all kinds of harsh environments throughout the world.