What is actually sliding?
Every rod end has a ball that turns in a race. The construction is defined by what sits between them, and that decides how the joint carries load and how it wears.
In a PTFE-lined rod end, a fabric or composite liner, usually PTFE-based and bonded to the race, is the sliding surface. The liner has a very low friction coefficient, so the joint runs without grease. The liner itself is the wear item. When it wears through, grease cannot restore it, because the liner was the thing that carried the film.
In a metal-to-metal rod end, a hardened ball slides on a hardened race. The lubricant carries the load between them, so the joint depends on grease staying in the contact. If the grease washes out or gets contaminated, the metal surfaces touch and gall. Metal-to-metal suits joints that are mostly static, need only periodic lubrication and must keep a minimum permanent set under high loads.
In a bronze-race rod end, the ball slides on a copper-alloy race. Bronze conforms to the ball well and tolerates some lubrication failure better than steel; aluminium-bronze races are less likely to seize or gall under vibration or poor lubrication. The price is capacity: about half that of a heat-treated steel race, by NHBB's figure. Bronze types vary by series, so check the construction and the lubrication instruction for the exact part.
Which duty points to which construction?
Work through the duty in this order.
Start with access. If the fitting cannot be reached without stopping the machine, a PTFE-lined rod end removes a recurring task. Where the joint is enclosed inside a guard or a packaging machine, grease near the process is also unwanted, and a liner avoids it.
Then load and shock. A heavy, steady load or a shocked, reversing load favours a metal-to-metal or bronze construction with a heavier body, but only if the grease is renewed. A light, slow oscillation is where a liner does its best work.
Then temperature. A PTFE liner works best from about −54 °C to 163 °C (−65 °F to 325 °F, QA1's stated range). Above that range, a greased metal construction is the usual choice, with a grease rated for the temperature (K02-Q1-12).
Then dirt and washdown. Grease attracts dirt, which abrades the sliding surfaces, and washdown can strip grease from a greased joint. In a wet or dirty place, a sealed or lined type can be easier to keep working (K02-Q1-11). A protected fitting and a post-wash regrease make a greased joint workable, but only if the routine is actually done.
Last, play. Every construction has a radial clearance. Most metal-to-metal bearings carry a small clearance on purpose, so that they do not bind when pressed into a housing. Clearance is a design choice, not a defect, but it also affects how the joint positions. A tighter joint positions better and may need more force to move. See Q1-09 for what play means when it grows.
What to watch for in each type
A liner wears slowly and then fails fast. Watch for play that grows, rising torque to move the joint, noise, and any visible displacement of the liner. Maintenance-free means no scheduled relubrication; it still needs inspection. Do not try to grease a liner unless its maker designed it for grease.
A metal-to-metal joint shows its problem as galling, dry grease at the seal, or wear marks on the race. The cause is nearly always lubrication, load direction or contamination (Q1-08).
A bronze race gives you warning in clearance growth and bronze wear. Check the maker's instruction for lubrication, because a bronze race is not automatically maintenance-free.
Two-piece swage-coined rod ends, a different construction, need separate care. Their ball-to-race fit is poor, so under oscillating loads they wear, fret and gall quickly.
Can you change the construction later?
Not safely in place. A different construction has a different load rating, a different lubrication need and often a different bore, so changing from one type to another is a redesign of the joint. The dynamic capacity of one construction must not be carried over to another. When you replace a joint, match the construction as well as the bore and the thread (K03-Q1-11).
Which construction survives mud, shock and roadside maintenance?
The choice follows the same order, but the duty points sit differently from a machine.
Dirt is the main enemy off-road. Mud, sand and dust get into the clearance, and a greased joint holds grit in the grease. Grit then scours the ball. A sealed or lined joint with a boot or cover keeps more of it out; a greased joint stays healthy only if it is cleaned and regreased after dirty use (Q1-08). A liner is not immune: grit pressed into it wears the ball just the same.
Shock favours the heavier constructions, and play shows. A landing or a hard hit is the shock load described above, and on a locating link it also reverses. A metal-to-metal or bronze-race joint on a heavier body copes with that more readily than a light liner, at the price of regular inspection; what decides it is a body and rating that cover the peak, not the liner alone. On a locating or steering link, the clearance of the joint is felt as a clunk or vague steering when the load reverses, so look at it early (Q1-09).
Roadside maintenance is hard. A joint that must be greased after every event is a recurring task; one that is lined, sealed and inspected at intervals set by the vehicle's designer may suit a vehicle that is used far from a workshop. Decide by load, dirt and who does the checking, not by the construction's name.
The construction and service interval for a steering or suspension joint are the vehicle designer's to set. The vehicle applications are in off-road rod ends and automotive and racing rod ends; the sister site syzmachine.com covers automotive suspension parts such as bushings and hub bearings.