What do the terms mean?
"Ball joint" is used for more than one part, so start by naming the three you will meet.
A rod end (spherical rod end) is a housing with a spherical bearing in it and a threaded shank. A pin or bolt goes through the bearing. Some catalogues also call it a ball joint, and some cylinder makers sell their rod ends as a "piston rod ball joint". An angle joint (DIN 71802) is a snap-on socket on a stud at 90° to the rod (Q1-02). An axial ball joint is a socket and stud in line with the rod.
How does the cylinder force travel through each one?
The path is different in each, and that decides the choice. The directions follow the table in Q1-07.
| Joint | Where the push-pull force goes | What the number to check is |
|---|---|---|
| Rod end | Across the bearing bore, along the shank. Makers rate it in kilonewtons as a static and a dynamic load (K03-Q1-07) | The rated load |
| Axial ball joint | The pull stroke pulls the stud out of the socket along its axis | The pull-off force, for the pull stroke |
| Angle joint | Across the stud, as shear and bending | The stud's bending and shear (Q1-07), not its pull-off figure |
How big is the cylinder force next to these limits?
Often larger. The theoretical force is pressure times piston area. At 6 bar (0.6 MPa), with no friction, and with typical rod diameters for each bore:
| Cylinder bore | Typical rod | Theoretical push force | Theoretical pull force |
|---|---|---|---|
| Ø10 mm | 4 mm | 47 N | 40 N |
| Ø12 mm | 6 mm | 68 N | 51 N |
| Ø16 mm | 6 mm | 121 N | 104 N |
| Ø20 mm | 8 mm | 188 N | 158 N |
| Ø25 mm | 10 mm | 295 N | 247 N |
| Ø32 mm | 12 mm | 483 N | 415 N |
These are theoretical values calculated as force = 0.6 × π × bore² ÷ 4 for the push, and 0.6 × π × (bore² − rod²) ÷ 4 for the pull, not catalogue figures.
For an axial ball joint, compare the pull force with its pull-off figure. Catalogue minimum values run from 30 N to 100 N, depending on size. From Ø16 up, the pull force is already above the largest listed pull-off. Even a force below the pull-off figure is not safe to run at: a pull-off figure is a retention check on a new part, not a working load (Q1-05).
For an angle joint, the pull-off figure is not the limit. The force bends the stud, so use the bending check in Q1-07. In that example, a Ø32 cylinder on an M8 stud with the force 10 mm from the support needs a stud of about 900 MPa yield at a safety factor of 5, beyond common studs, and the answer is the same: a rod end.
ISO 8139 bases the design of rod-end spherical eyes on the maximum force resulting from the cylinder bore and pressure.
Which standards apply?
ISO 8139 is the one written for air cylinders (check the current edition before you quote it). It describes rod-end spherical eyes designed for 1,000 kPa (10 bar) series cylinders to ISO 6432 and ISO 15552, and adds that this "does not limit their application". They are used on piston rods of pneumatic cylinders to transmit the cylinder force under oscillating and tilting movements.
ISO 12240-4 is the standard for spherical plain bearing rod ends in general. It sets dimensions, tolerances and radial internal clearances (ISO 12240-4). Older catalogs list the same parts as "DIN ISO 12240-4 (DIN 648)". Neither standard gives load ratings or service life; those come from the maker (K03-Q1-29).
DIN 71802 covers angle joints and sets dimensions only (Q1-02).
How do tilt, play and upkeep compare?
| Rod end | Angle joint | Axial ball joint | |
|---|---|---|---|
| Tilt | Depends on series and maker; 19° is listed for one cylinder rod end | 15° with clip, 18° without (Ganter's catalogue, also sold as JW Winco) | Maker-specific; ±18° is listed by one maker |
| Play | Has a defined radial clearance | Has play; a smooth-running version has more | Has play |
| Upkeep | Greased, or maintenance-free with a PTFE liner | Greased metal seat, optional cap | Greased metal seat |
| Quick release | Pin must be removed | Some series come apart by hand | Some series come apart by hand |
Steel-on-steel spherical plain bearings always run with some clearance, so the clearance has to be allowed for at the point where the cylinder reverses. A PTFE-lined rod end can be made with little or no clearance (K01-Q1-04).
Does a spherical joint fix side load?
No. Both ends should let the cylinder find its own line, so that the force acts along the cylinder axis. If the force acts along the line through both pivot centres, the joints add no side load. A joint absorbs angle. It does not carry a large sideways force, so guide the load instead (Q1-07).
Is there a middle option?
Yes: a rod end with a threaded stud pressed into the inner ring, used like an angle joint. Some rod ends can be supplied that way. This gives the stud-style mounting of an angle joint with the load path of a rod end. Ask the maker for the stud's own bending rating (K11-Q1-07).
If the motion stays in one plane, a clevis is another choice (K05-Q1-03).
How do you choose?
If the cylinder swings and carries the load, or if the force fails the check for the joint (the pull force against the pull-off of an axial joint, or the stud bending of an angle joint), use a spherical rod end. If the force is small, the link is for control or positioning, and you need quick release or want to save a bracket, a ball joint can do, checked with the method above and a wide margin. If you want a stud mounting but need rod-end capacity, use a rod end with a threaded stud. If the motion stays in one plane, use a clevis.
A compact ball joint is not a substitute for a rated rod end, as SYZ's own advice on small ball joints also says.
FAQ
Is a ball joint the same as a rod end?
Which suits a pneumatic cylinder piston rod?
Does ISO 12240-4 give a load rating?
Related reading
- Q1-02 Forms C, CS, B and BS
- Q1-05 Pull-off force and vibration
- Q1-07 Sizing a small ball joint
- K03-Q1-02 Types of rod ends and rod eyes
- K03-Q1-29 Standards for rod ends
- K05-Q1-03 Clevis or spherical rod end
- syzmachine.com: Ball Joint vs. Heim Joint vs. Spherical Bearing