A revolute joint permits rotation about one axis while constraining translation. A cylindrical journal needs axial retention to serve as this one-freedom joint; a smooth unretained shaft and sleeve can also slide. The drawing includes collars and a fixed bearing support.
Replacing manual motion in Cylindrical Pair (revolute) Mechanism
If cylindrical pair (revolute) mechanism is converted to actuator drive, the actuator is no longer just a replacement for a crank or cam. It becomes part of the timing, force, and limit-control system, so the travel must be checked through the full motion.
For an actuator-driven pivot, resolve the linkage forces to find the radial load carried by the journal. That bearing reaction can differ from the actuator’s axial force.
A crank or cam may tolerate continuous rotation through a difficult part of the cycle, while a linear actuator has defined end positions and internal limits. That difference matters when the mechanism needs repeatability or must not drive into a hard stop.
- Map actuator stroke to the actual output travel before choosing a model.
- Check for binding, changing leverage, and peak load near the ends of travel.
- Decide whether timed motion is acceptable or position feedback is required.
For a FIRGELLI design review, calculate the load case with the FIRGELLI actuator force calculator and plan the switching or controller arrangement with the linear actuator wiring diagram generator. If an actuator or cylinder is already installed, the linear actuator replacement finder helps keep the comparison tied to real dimensions.
Cylindrical Pair Revolute Interactive Calculator
Inspect a retained rotating journal. Adjust radial load, pin diameter and effective bearing length to calculate nominal projected bearing pressure.
Equation Used
- Radial load carried over the entered effective journal length.
- Projected rectangular area D×L, rather than cylindrical surface area.
- Ideal axial retention leaves rotation only; no thrust-load rating is calculated.
- Illustrative rotation rate; no speed-dependent pressure or thermal model.
Nominal projected pressure is not peak local contact pressure. No bearing material or allowable stress is assumed.
Rotation with axial retention
The pin turns inside a stationary sleeve. Collars at both ends prevent the pin from sliding axially. The upper half of the bearing is cut away so the journal and its end-face marker remain visible.
The load arrow points radially toward the journal. Diameter and effective bearing length change the drawing proportions. Rotation illustrates the remaining freedom; it does not imply frictionless performance in a real bearing.
Compare journal size and radial loading
The calculator is useful for the nominal pressure check of a pin or plain journal bearing. The supplied load is treated as the radial load carried by this bearing, not a remote force whose reaction must first be resolved through an arm or multiple supports.
Use effective loaded bearing length rather than overall pin length. Collars in the picture represent axial restraint; their thickness and shape are illustrative.
Projected bearing pressure
Projected area is the rectangle formed by diameter D and effective length L: A=D×L. It is not the curved surface area πDL. With D and L in mm and radial force in N, F/A gives N/mm², numerically equal to MPa.
The pressure conversion uses approximately 145.03774 psi per MPa. The length-to-diameter output is L/D. Neither result establishes an allowable load or material safety factor.
An 18 kN load on a 40 mm by 60 mm journal
Projected area is 40×60=2400 mm². The 18,000 N radial load gives nominal projected pressure 7.5 MPa, or approximately 1087.8 psi. The bearing length-to-diameter ratio is 1.5.
Doubling effective bearing length to 120 mm halves this nominal pressure to 3.75 MPa at the same diameter and load. This arithmetic does not guarantee that a real longer bearing distributes load uniformly.
What nominal pressure leaves out
Real local pressures depend on clearance, compliance, misalignment and lubrication. Speed and duty cycle affect frictional heating. Pin bending, bearing wear and axial-thrust restraint require separate checks.
Allowable bearing pressure depends on the selected material and operating conditions. Nominal pressure alone does not establish suitable clearance, temperature limits or service life.
Revolute-journal questions
Why add collars?
They illustrate the axial constraint needed for a revolute joint. Without axial retention, a smooth cylindrical pair can slide as well as rotate.
Why is area a rectangle?
The conventional nominal bearing-pressure measure uses projected diameter times length.
Does the pressure change as the marker rotates?
No. The entered radial load and circular journal dimensions remain constant.
Can the result prove the bearing is safe?
No. It is a nominal geometric pressure calculation without a selected bearing material, speed, temperature or life model.
References
Lynch and Park, Modern Robotics, section 2.2, revolute and cylindrical joint freedoms. Thordon Bearings Engineering Manual, design guide, bearing-pressure subsection, radial load divided by projected diameter×length and the distinction from actual pressure distribution. No Thordon material limit is assigned to the illustrative bearing.
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