How do you decide the interval?
Start with the maker's instruction. If the manual gives one, use it. If it does not, judge it from three things.
The load. A link that carries high, shock or reversing loads, or whose failure would stop the machine or hurt someone, is checked more often than a lightly loaded one. A link that goes from pull to push every cycle is a fatigue case and deserves closer attention (Q1-02).
The environment. Vibration, washdown, salt, dust and heat all shorten the interval. Moisture and cleaning chemicals attack the threads and the crevices around nuts and pins (Q1-08). Dust sticks to grease and wears the thread.
The maker. A maker may give an interval for a rod end, a grease or a coating, and these override any rule of thumb.
Standards for lifting and rigging hardware, such as ASME B30.26, set strict inspection intervals and removal limits for that hardware. They are written for lifting equipment and for lifting loads, not for machine linkages, so do not transfer their intervals or limits to a machine link. The same goes for the data of lifting screw jacks, which are a different product (Q1-01).
What should you check on a machine linkage?
- Length. Has the centre-to-centre length drifted from the set value? Measure it as in Q1-04. A change means the thread has turned or something has worn.
- Jam nuts. Is each one tight, on the right end, and is the paint or marker line across the nut and the body still unbroken? A gap in the line means it has turned (Q1-05).
- Play. Push and pull each joint by hand and measure the lost motion (Q1-11). Rod end and pin play grows with wear (K03-Q1-22, K02-Q1-14).
- Threads. Look for damage, galling, and corrosion at the thread ends and under the nuts. A thread that turns stiffly where it used to turn freely can hide corrosion or damage (Q1-09, Q1-08).
- Engagement. Check that enough thread is still in the body at the current setting: the maker's figure, or as a starting point about 1 × the thread diameter in steel, with more in a softer part (Q1-04). Many tube ends have an inspection hole for this (Q1-10). A tube body can hide how much is engaged.
- The body and tube. Bends, cracks, dents or stretched sections, especially near the ends and at any weld.
- Rod end clearance at the ends of travel. Run the mechanism through its full travel and check that nothing touches or binds at either extreme (Q1-10).
How often should you lubricate?
It depends on the part and on the maker's instruction. A thread that is adjusted from time to time can carry grease or an anti-seize compound to prevent seizing, reduce wear and ease later adjustment (Q1-09). Do not over-lubricate in dusty places, where excess grease collects grit. Check the compound against the metals and the environment. A rod end that needs grease follows its own interval (K02-Q1-08, K01-Q1-08).
When must you replace it?
When the part can no longer be trusted to carry the load it was chosen for. These conditions mean replacement:
- A bent, twisted, stretched or cracked body, rod or tube. Do not straighten it.
- Damaged or worn threads, or a thread that has galled, even if it came free (Q1-09).
- Deep pitting or corrosion that reduces the section, or corrosion that has got into the thread.
- Heat damage, weld spatter or arc strikes.
- Signs of unauthorised welding or modification.
- Rod end play or wear beyond what the maker permits (K03-Q1-22).
- Anything else that gives you doubt about continued use.
For limits such as how much wear is too much, use the maker's data. Without it, do not invent a figure; replace the part.
What should you add to the checks on a vehicle?
Steering and suspension links are safety parts, so the vehicle maker's service schedule or the event organiser's current rules set the intervals; there is no general figure. What a vehicle adds is events. Check the links after any hard hit (a rock, a kerb, a landing) and whenever the vehicle handles differently. Sight along each link for a bend, look for bright rub marks showing that something is touching, check the boots and seals on the rod ends, look at the weld areas on a tube link for cracks, and check the nut marks from Q1-05. A bent link is replaced, not straightened, as above. Wear limits come from the maker; if the maker gives no limit, treat measurable play or any damage as the reason to replace.