Tie-rod vs welded body cylinder construction: pros, cons, and use cases

The cylinders with tie rods employ rods with threaded ends that clamp the end caps onto the barrel, which makes them field-serviceable and perfect for industrial applications using standardized pressure ratings of around 3500-3500 PSI. While welded body cylinders feature end caps that are permanently attached to the barrel. They offer a smaller footprint, greater capacity for pressure, and greater resistance to side loading and contamination However, they will need to be repaired by a factory in the event that seals or components fail. The best choice is based on whether durability or serviceability is more important for the particular application.

Making the choice between a tie rod and body construction that is welded is among the primary options engineers have to make when designing the hydraulic cylinder they want to use, and it can have a direct impact on maintenance strategies and the cost of ownership and the reliability of the system. Both models perform the same fundamental task—the conversion of the energy of hydraulics into linear force; however, they accomplish this through fundamentally different philosophies of mechanical design.

How do tie-rod cylinders get made?

A cylinder made of tie rods is sandwiched between two caps at the end that are held in tension by an external set of steel rods that run across the distance. Rods can be torqued up to the preload specified, which ensures the cylinder is in place against internal pressure, with no welding.

The origins of this construction date back to NFPA (National Fluid Power Association) interchangeable dimensional standards, which is the reason tie-rod cylinders often are referred to as "NFPA cylinders." Since their dimensions for mounting and bore sizes, as well as stroke lengths, are based on the same envelopes, a tie rod cylinder made by one manufacturer is often swapped for a unit from another manufacturer without having to redesign its mounting interface.

The advantages of tie rod construction

• Complete field serviceability. Because the end caps don't get attached, technicians can take apart the cylinder on the spot and replace worn seals, piston rings, or rod bearings and then reassemble the unit without having to return the machine to a shop.
• Standardized interchangeability. Dimensions that are NFPA-compliant mean that replacement cylinders can be purchased from multiple vendors, reducing the risk of downtime when a unit is damaged.
• Lower repair cost. Seal kits are cheap and readily available. Rebuilding labor is simple in comparison to the specialized repair of welded bodies.
• Access to visual inspection. Tie rods are a great way to measure preload torque and visually verify the integrity of the assembly when performing regular maintenance.

Con-siderations of tie-rod construction

• Lower pressure ceiling. The standard tie-rod cylinders are built to last for continuous use at 3,000 to 3,500 PSI. Beyond that, the diameter of the tie rod and preload requirements begin raising costs and bulk in a significant way.
• Larger envelope. The rods on the outside provide length and width to the design in comparison to a welded model with the same bore and stroke, which is important in the case of space-constrained mobile equipment.
• Preload sensitivities. If the torque of the tie rod decreases in time because of the cyclic or vibrational load The seal on the end cap will begin to weep, and the uneven torque across rods could cause tension that is not aligned to the barrel.
• Additional failing points. The tie rod, nuts, and washer are separate components that are susceptible to loosening or corroding during the life of the cylinder.

How are welded body cylinders made?

Body cylinders that are welded eliminate tie rods that are externally attached. The ends are welded directly onto the barrel, creating an all-in-one design. Certain designs feature fully welded bodies with a movable gland to allow rod seal access, while fully sealed designs need no access points from outside whatsoever.

This kind of structure is prevalent in mobile hydraulics—excavators, loaders and dump trucks, cranes, and agricultural equipment—where the compact design and the durability of high-cycle cycles in harsh conditions are more important than the ease of field reconstructions.

The advantages of welding body construction

• More pressure capacity. Welded cylinders are able to handle pressures up to 3000 PSI and include a variety of models that are mobile and heavy-duty, designed for 5,000 PSI or more, as the joint is welded and is more efficient at distributing stress than tie-rods' individual clamping points.
• Compact footprint. Without rods, the cylinder's outside diameter is closely matched to the barrel's diameter, which is important when the space for packaging is limited, for example, in an excavator boom or an arm for a compact loader.
• A better side-load and shock resistance. The barrel-to-cap joint welded by welding resists the torsional and bending stress typical in mobile equipment more effectively than an unbolted or preloaded assembly.
• The elimination of leak routes. A smaller number of mechanical joints means fewer locations for hydraulic fluid to pass through in time. This is beneficial in situations where pollution from the environment is a problem.
• Better contamination resistance. The smooth, sealed exterior spits out dirt, mud, and debris faster than a tie rod's exposed rod and nut assembly, which is a significant benefit in agricultural and construction areas.

The cons of body construction with welded joints

• It is either impossible or difficult to repair in the field. Damaged seals, a scored barrel, and a bent rod will typically require sending the entire cylinder to an expert rebuilder with the ability to machine and weld to fix it or replace it entirely.
• Cost of replacement is higher. Since repair is a complex process, most operators just replace faulty welded cylinders rather than rebuilding them, which increases the cost long-term if the failure rate is high.
• Custom-designed dimensions. The cylinders made of welds are often constructed on a custom basis for an OEM product rather than an interchangeable standard. This may delay the time to replacements.
• Access to inspection is limited. Without external tie rods or an easily removable gland, verifying internal condition without disassembly is harder, which pushes maintenance toward a run-to-failure or scheduled-replacement model rather than incremental servicing.

Selecting the appropriate construction for your needs

When tie-rod cylinders are logical

Tie-rods are the best option for stationary and industrial equipment where frequent maintenance access is expected, and standard replacement parts are important. Common use cases are:

• Presses hydraulic and equipment for stamping
• Automation for manufacturing and actuators for assembly lines
• Material handling systems for fixed installations
• Lab equipment and test benches in which cylinders can be regularly rebuilt
• Examples where a maintenance staff is looking to keep spare seal kits in stock instead of spare pistons

When welded body cylinders are used, they make sense.

Welded construction is the most common option for off-highway and mobile equipment that has to endure shock loading as well as vibration and exposure to environmental elements. Common applications include:

• Wheel loaders, excavators and backhoes
• Tractors, agricultural equipment and combiners
• Hoists for dump trucks and refuse vehicles
• Material lifting equipment and cranes
• Any place that exposes the cylinder to dirt or water, mud or road debris

The process of making a decision: the key selection elements

If you are deciding between two constructions, consider the following:

1. Operating pressure. If the system is operating at or above 3,500 PSI often, then welded construction is typically the most secure option for engineering.
2. Maintenance is a philosophy. Facilities that have in-house hydraulic technicians benefit from the serviceability of tie rods Operations that outsource repairs or prefer swap-and-replacement methods may be better handled by weld units.
3. Space limitations. Applications for mobile and compact equipment tend to favor smaller-sized welding designs.
4. Environmental exposure. The presence of water, dirt, or abrasive conditions favors the exteriors of welding cylinders.
5. The total cost to own. Cylinders with tie rods typically have a chance to win on the lifetime rebuild cost, and welded cylinders typically have a better durability and decreased downtime caused by leaks.

1. Can a cylinder with a tie rod be changed to a welded-body design and reverse the process?

No. The two models use fundamentally different manufacturing techniques and load paths for structural loads; therefore, conversion is not feasible. Replacing one design with another is a matter of choosing a cylinder that is specifically engineered specifically for the model from the start.

2. Which one is more cost-effective over the time span?

It's dependent on how often and what the duty cycle is. Cylinders with tie rods usually come with lower repair costs per repair, which makes them more affordable for seal replacements that are routine. The cylinders that are welded cost more to repair or replace but generally fail less often in the harsh environments of mobile, which may help offset the higher cost.

3. Do welded body cylinders designed to withstand higher pressures than tie rod cylinders?

Yes, there's some overlap. Heavy-duty tie-rod cylinders made of reinforced tie rods are able to reach more pressures than the standard models; however, welded construction can more effectively handle high-pressure stresses without the weight added by larger tie rods.

4. What can I do to determine if the seals on my tie-rod unit require replacement or the whole unit?

The signs of external weeping at the ends of caps and a slow drift of the cylinder during load or scuffed rod surfaces typically indicate an issue with the seal kit rather than a complete unit failure. A hydraulic technician is able to identify this by visual inspection as well as the pressure drop test.

5. Why do manufacturers of mobile equipment mostly rely on the welded body cylinders?

Cylinders for mobile equipment are subject to continuous shock loads and vibration as well as side loads caused by the arm and boom geometry, as well as the exposure to water and dirt. Welded construction withstands these stresses better than bolted assemblies, and the smaller envelope is able to fit the stricter requirements of packaging for booms and arms as well as lift mechanisms.