Consider Wear-Ring Options
EP Editorial Staff | September 21, 2018
Selecting the best non-metallic wear ring for an application can greatly increase its effectiveness.
Whatever the material, wear rings can be key components in a hydraulic system. They absorb the side-load forces in a piston and/or rod hydraulic cylinder, while eliminating metal-to-metal contact. In general, wear rings can extend the life of a hydraulic system. Moreover, it’s faster and less expensive to replace a wear ring than an entire cylinder.
Although there are applications where metal wear rings are appropriate, there are numerous benefits of using non-metallic versions in fluid-power applications such as off-highway, material-handling, industrial machines, mobile hydraulics, and agricultural equipment. The benefits include:
• favorable friction behavior
• high load-bearing capacity
• easy installation
• damping of mechanical vibrations
• good wiping effect
• protection of seal against “dieseling” (hydraulic fluid flowing too quickly across the cylinder)
• elimination of local stress concentrations
• reduced downtime, due to less damage to rods or pistons, and longer product life.
Non-metallic wear-ring materials include polytetrafluoroethylene (PTFE)-based compounds, as well as thermoset and thermoplastic options. Each has its own specific characteristics.
PTFE-based wear rings provide extremely low friction within systems and are particularly well suited in dry-running applications. They can be mated with steel, aluminum, and cast iron. The smoothness of PTFE-based wear rings means such little abrasion takes place that, in some cases, less-expensive hardware can be used in a system.
Additionally, PTFE-based rings, due to their low friction characteristics, help reduce the incidence of stick-slip when starting up the system.
Common uses of PTFE-based wear rings include equipment operating at high temperatures and pressures.
Thermoset wear rings have outstanding wear-resistance properties and are designed to offset heavy loads at low speeds. They
address application demands for strong, light, durable rings. Although thermoset rings are dense, they are elastic in comparison with metal rings.
Thermoset options are available in diameters as large as 60 in. Larger diameters can be manufactured from strip. Most of these rings can be mated with steel, hard chrome-plated surfaces, and cast iron. Example applications include tensioner cylinders, injection-molding hydraulic systems, and aerospace landing gear.
Thermoplastic wear rings are the workhorses of hydraulic systems. In many systems, they are temperature resistant and can provide a high level of performance at a more affordable price than PTFE-based or thermoset wear rings. Most can be mated with steel, hard chrome-plated surfaces, and cast iron.
Internal valves and compressor components are two popular applications for thermoplastic rings.
Choosing the best material for your application is a matter of evaluating the type of application, the size of the load within the system, the installation space, and the mating surface.
Wear rings typically are supplied as split rings or strip material and offered in a roll or precut to size. Some feature chamfered edges that facilitate installation and prevent edge forces from becoming too high in the corner radii of the grooves. There are several types of split designs:
• Scarf cuts (angle cuts) are standard for hydraulic applications. The angle reduces the edge loading on the ends of the ring and is preferred for linear systems.
• Straight (butt) cuts are often seen on strip or net molded rings and are most often used in rotary applications.
• Step cuts provide the best wear-ring coverage and can act as a buffer against pressure spikes, but usually at an additional cost.
Since thermoplastic rings are injection molded, it is not uncommon to see parts shrink unevenly during the cooling process, especially if the tooling design is not optimal. The shrinkage can cause a shape that’s thinner in the center than on the ends—often referred to as dogboning.
One way to quickly test a wear ring for dogboning is to lay it against a flat surface and visually assess it for daylight shining through. Of course, dogboned wear rings can be easily discovered during prototype tests. In extreme cases, dogboned rings cannot be installed. In less extreme cases, they cause higher friction than expected and/or act as a wiper, so hydraulic fluid doesn’t flow properly. EP
Information in this article was provided by Tom Zozokos of Trelleborg Sealing Solutions, Streamwood, IL, and Beth Figliulo of Trelleborg Sealing Solutions Americas, Fort Wayne, IN. For more information on wear rings, visit tss.trelleborg.com.
Understand Wear-Ring Performance
Some wear rings are made for volume commercial applications. This can mean, in certain circumstances, that the tolerance to which they are machined may be wide, which typically could be as much as 0.005-in. of the total wall.
Precision machining produces wear rings with tight clearances and less tolerance buildup. In terms of total cost of ownership, the extra expense associated with such wear components can, in some applications, pay dividends, i.e., within systems that will be marketed as high-performance and judged on their long service life. For systems that aren’t expected to be used over long periods of time, cost savings may be achieved by specifying larger-tolerance commercial rings rather than precision types.
In either of these cases, the quality of the injection molding and, in turn, consistency in the wear-ring’s performance, is an important aspect. After all, consistency in wear-ring performance means faster installation and increased service life between changeovers.