What does the piston rod thread look like on paper?
A diameter, a thread count or pitch, and a length. Peninsular Cylinders gives an example of a male rod thread, 3/4"-16 x 1.12": 3/4" is the diameter of the threaded end, 16 is the number of threads per inch, and 1.12" is the overall length of the thread. For a female thread the same numbers mean the inside diameter of the hole, the threads per inch, and the minimum depth. Metric rods are written with a pitch, such as M12 × 1.25 (K06-Q1-07).
The clevis must be the opposite: a rod with an external thread takes a clevis with an internal thread, and a rod with an internal thread takes a clevis with a male stud (see the FK and FT types in Q1-01).
Is the rod thread standard?
For pneumatic cylinders, often. The National Fluid Power Association (NFPA) assigns a set of standard rod-end thread styles to each piston rod diameter, so that cylinders from different makers are interchangeable; a cylinder made with another thread is not interchangeable. ISO 8140 names ISO 6432, ISO 15552 and ISO 21287 as the cylinder standards its clevises are designed for (Q1-02). The thread is set by the piston rod, not by the bore, so read it from the cylinder maker's data or from the rod itself, and quote it in full on the order, including the hand.
What must match the machine?
Each of these sets either the fit or the geometry, and a wrong pin size, wrong width or loose bracket fit can cause as many problems as a wrong bore or stroke:
| Dimension | Why it matters |
|---|---|
| Pin diameter | Fit, shear capacity and wear life (Q1-06) |
| Inside width between the lugs | Must match the bracket or mating tab thickness |
| Lug thickness | Strength around the pin hole |
| Pin centre distance | Sets the machine geometry and stroke position |
| Rod thread or weld connection | How the clevis joins the rod |
| Pin retention | How the pin is held in operation (Q1-07) |
| Bushing or sleeve | Can improve wear life in repeated duty (Q1-12) |
A threaded clevis suits a connection that must be removable for service or adjustment, and a welded clevis is built into the rod end structure and suits custom or heavy-duty designs, with weld quality and dimensional accuracy then needing careful control.
How do you measure a clevis you are replacing?
Take six measurements:
- The pin diameter of the existing connection.
- The inside width between the lugs.
- The pin centre distance the machine geometry needs.
- Whether the rod end uses a threaded or a welded clevis.
- The closed and extended length of the whole cylinder.
- The bore, stroke and port orientation, if space is tight.
A small difference in width, pin size or centre distance can still cause installation problems when the old cylinder "looks similar". Measure the rod thread on the rod, not on the old clevis, because the old internal thread may be worn or damaged (K03-Q1-11).
How far must the clevis screw on?
Far enough that the thread does not strip before the rod gives way. A bolted joint should be designed so that the screw breaks before the threads strip. A common rule of thumb for minimum engagement is about one times the nominal diameter in steel and two times in aluminium (Nord-Lock), and it is not enough in softer material or special alloys.
You can see where the figure comes from with a screening calculation. Take an M12 × 1.25 rod thread. Its pitch diameter is 11.188 mm and its minor diameter 10.466 mm, so the tensile stress area is π ÷ 4 × ((11.188 + 10.466) ÷ 2)² = 92.1 mm², and the rod thread can carry 92.1 mm² times its tensile strength. The thread strips over an area of about 0.5 × π × d × Le, where Le is the engagement length, at a shear strength of about 0.6 of the tensile strength. Setting the two equal, for the same material on both sides:
Le = 92.1 ÷ (0.3 × π × 12) = 8.1 mm, which is 0.68 times the diameter.
If the softer part has half the strength, as an aluminium or soft-steel clevis may against a hardened rod, the engagement doubles to 16.3 mm, 1.36 times the diameter. So "one diameter in steel, two in aluminium" sits above these figures and covers partial first threads, tolerances and some scatter. It is a simplified model, so for a part that matters take the engagement from the maker's data. If the rod and clevis are of different metals, the softer one sets the engagement (K04-Q1-23).
How do you set the length?
By turning the clevis on the rod, within the range the engagement allows. That range is smaller than people expect. Suppose the rod thread is 32 mm long, the clevis thread is 20 mm deep, and the required engagement is one diameter, 12 mm. The clevis can screw on 20 mm at most. Backing it out until only 12 mm is engaged gives an adjustment range of 8 mm. A jam nut on the rod thread takes up some of the thread length as well, so count it.
Check that the engagement is still enough at the longest setting you will use (K06-Q1-04). Do not set the length by backing the clevis out beyond what the thread depth permits. If you need a longer reach, use a different rod end or an adjuster, and do not rely on a few threads.
How do you lock it?
Run a locknut down against the clevis end face once the length is set, with the same thread hand as the rod. A locknut locks the thread, and it does not give structural support, so the required engagement must still be there. Under a pull it does not carry the load. Under a push or reversing load it shares the load, so it has to be tightened to a real preload, and the right value comes from the part's maker (K11-Q1-05, K06-Q1-05).
Other measures exist:
- Thread-locking adhesive. A medium-strength grade that is removable with hand tools (Loctite 243 is one) suits a clevis you may adjust again.
- A set screw. Some clevis variants carry an additional threaded pin to DIN 913 / ISO 4026 that locates the clevis against rotation.
After setting and locking, check that the length has not changed and that the clevis does not turn by hand. Check it again after the machine has run, since vibration is what loosens it.
What goes wrong with a wrong or loose clevis?
A wrong or loose clevis shows up as loose fit and hole wear from a wrong pin diameter, impact loading from too much clearance between bracket and clevis, side load passed to the rod and seal area from poor alignment (Q1-03), premature wear of pins and bushings, and cracking or deformation around the lugs under repeated shock. A generic clevis with no rated load and a soft pin tends to fail in the same sequence: the thread loosens, the pin bore elongates, and the body cracks at the thread root. A clevis with a stated rating and a hardened pin is the safer choice.