A half-yoke connection is a single-eared clevis fitting on a hydraulic cylinder rod end that engages a matching double-eared bracket through a single shear pin. The pin is the critical component — it carries the entire cylinder load in double shear and locates the joint angularly. The design lets the cylinder pivot freely as the driven member arcs, so rod-end side loading stays minimal. You see it on excavator buckets, dump trailer rams, and ISO 6020 industrial cylinders pushing 20-tonne loads.
Half-yoke Connection Interactive Calculator
Vary cylinder load, pin diameter, pin material strength, and shock factor to see double-shear pin stress and utilization.
Equation Used
The half-yoke pin is checked as a double-shear joint. The axial cylinder force is multiplied by the shock factor, then divided across two shear planes using tau_pin = 2F/(pi d^2). The allowable shear stress is estimated as 0.4 times the pin yield strength.
- Pin carries the cylinder load in double shear.
- Load is axial through the half-yoke joint.
- Allowable shear stress is estimated as 0.4 times pin yield strength.
- Pin diameter is measured at the shear planes.
How the Half-yoke Connection Works
A half-yoke connection works on one principle — keep the cylinder rod loaded purely along its axis. Hydraulic rods bend and gall fast under side load, so the joint at the working end has to let the cylinder swing freely as the load travels through its arc. The half-yoke (a single ear with a bored hole) drops into a matching double-eared clevis bracket on the driven member, and a hardened pin passes through all three ears. The cylinder now pivots on that pin like a hinge, transferring force as a clean tension or compression along the rod centreline.
The pin and bushing geometry is where most builds go wrong. The pin must be a snug sliding fit in the hardened bushings — typically H8/f7 — with no more than 0.05 mm radial clearance on a 25 mm pin. Open it up to 0.2 mm and you get hammer loading every time the cylinder reverses, and the bushing ovalises within a few hundred cycles. The single-ear yoke must also be thick enough to take the bearing pressure from the pin without dishing — that bearing stress, σb = F / (d × t), needs to stay below roughly 80 MPa for steel-on-steel running joints and below 25 MPa if the pin and bushing are dry.
Failure modes are predictable. Pin shear failure looks like a clean 45° fracture face — that means you sized the pin diameter wrong for the rated cylinder thrust. Wallowed-out bushing bores with bright wear scars mean the clearance was too loose or the joint ran without grease. A cracked yoke ear, splitting outward from the pin bore, means the ear wall thickness was undersized — for a clevis pin joint the rule of thumb is ear thickness ≥ 0.5 × pin diameter, and the side wall outboard of the pin hole ≥ 0.4 × pin diameter.
Key Components
- Single-ear yoke (rod-end clevis): Threads or welds onto the cylinder piston rod and presents one bored ear to the mating bracket. Bore is typically reamed H8 to match the pin, with ear thickness 0.5-0.7 × pin diameter. On a 50 mm bore ISO 6020 cylinder you would expect a 25 mm pin bore in the yoke.
- Mating double-ear bracket: The fixed half on the driven member with two parallel ears that straddle the yoke. Inside spacing matches yoke thickness within +0.2/-0 mm so the joint cannot rattle laterally. Bracket ears must be coplanar within 0.1 mm or the pin binds.
- Shear pin: The single load-carrying member. Hardened to 50-55 HRC on a tough core, sized so calculated shear stress stays under 80 MPa for normal duty. Retained by a cotter, snap ring, or threaded end-cap depending on duty cycle.
- Hardened bushings: Pressed into the yoke and bracket ears to give a renewable wear surface. Typical material is oil-impregnated bronze (SAE 841) or hardened steel with a grease groove. Replacing a £4 bushing is faster than re-machining a £400 yoke.
- Grease fitting and cross-drilled pin: Most production half-yoke joints use a Zerk fitting in the pin head feeding a cross-drilled passage that delivers grease to the bushing-pin interface. Without it, dry running drops bushing life from 5,000 hours to under 200.
Real-World Applications of the Half-yoke Connection
You find half-yoke connections wherever a hydraulic cylinder drives a member that travels through an arc — which is most heavy machinery. The reason is purely mechanical: any cylinder mounted rigidly at both ends would bind or buckle the rod the moment the driven member moved off its starting geometry. The half-yoke gives that single degree of rotational freedom, lets the cylinder track the arc, and keeps the rod in pure axial loading where it belongs.
- Construction equipment: Caterpillar 320 excavator boom and stick cylinders use half-yoke rod ends pinned into welded brackets on the boom — pin diameters run 70-80 mm carrying cylinder thrusts above 250 kN.
- Agricultural machinery: John Deere 8R series tractor three-point hitch lift cylinders connect to the rockshaft arms through half-yoke fittings, allowing the cylinders to swing as the lift arms arc through 90°.
- Material handling: Hyster forklift mast tilt cylinders use half-yoke rod ends to the mast frame so the cylinder pivots cleanly as the mast rakes forward and back through 15°.
- Truck and trailer: Mailhot dump body hoists pin into the subframe with a single-ear clevis at the rod end, sized for 50-tonne payload tipping loads.
- Industrial automation: Parker and Bosch Rexroth ISO 6020/2 standard cylinders ship with optional MP3 single-ear rod clevis to mate with customer-supplied double-ear brackets on press tooling and clamping fixtures.
- Marine deck equipment: Rapp Marine trawl winch luffing cylinders run half-yoke connections at the rod end so the cylinder follows the boom arc through full lift travel under saltwater wash-down conditions.
The Formula Behind the Half-yoke Connection
The single design calculation that decides whether a half-yoke connection survives is the pin shear stress check. The pin sits in double shear — the cylinder load splits between two cross-sections, one through each bracket ear. At the low end of the typical operating range (cylinder loaded to 25% of rated thrust) the pin barely notices it. At the nominal rated thrust the pin should be running at roughly 40-50% of allowable shear stress, leaving headroom for shock loading. At the high end — peak relief-valve pressure with shock — you can hit 90% of allowable, which is where pin selection separates a 10-year joint from a one-season joint.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| τpin | Average shear stress in the pin (double shear) | MPa (N/mm²) | psi |
| F | Cylinder axial thrust at the rod end | N | lbf |
| d | Pin diameter at the shear plane | mm | in |
| τallow | Allowable shear stress for pin material (typically 0.4 × yield) | MPa | psi |
Worked Example: Half-yoke Connection in a vineyard grape harvester shaker-head lift cylinder
You are sizing the half-yoke pin for the shaker-head lift cylinder on a New Holland Braud 9090X grape harvester being rebuilt at a vineyard service shop in Mendoza Argentina. The cylinder is 80 mm bore running at 210 bar working pressure, lifting the picking head through a 0.6 m arc. The pin will be 4140 steel hardened to 45 HRC with allowable shear of 240 MPa.
Given
- Cylinder bore = 80 mm
- Working pressure = 210 bar
- Relief pressure (peak) = 260 bar
- Pin material allowable τ = 240 MPa
- Trial pin diameter = 30 mm
Solution
Step 1 — calculate cylinder thrust at nominal working pressure. Push area on the cap end is π × (80/2)2 = 5026 mm2, and 210 bar = 21 N/mm2:
Step 2 — calculate shear stress in the 30 mm pin at this nominal load:
That is 31% of the 240 MPa allowable — well inside the safe zone. The joint feels solid, the pin barely warms, and bushing wear stays linear over thousands of cycles.
Step 3 — check the low end of the typical operating range, an unloaded head lift at roughly 80 bar standby pressure:
At 12% of allowable the pin is loafing. This is the regime where pin diameter is overkill for stress but still required for bearing area in the bushing — you cannot just shrink the pin because bushing pressure σb = F / (d × t) sets a separate floor.
Step 4 — high-end check at 260 bar relief with a 1.5× shock factor (the head catches a vine post and spikes the circuit):
That is 58% of allowable — still safe, but only because you sized the pin generously. Drop to a 25 mm pin and τhigh jumps to 200 MPa, and any further shock pushes the pin into plastic deformation. That is how you end up with a bent pin you cannot drift out in the field.
Result
The 30 mm 4140 pin runs at 74. 6 MPa nominal shear stress — comfortably inside the 240 MPa allowable with a safety factor of 3.2. At 80 bar standby the pin is barely loaded at 28 MPa, but you cannot downsize it because bushing bearing area sets the minimum diameter; at 260 bar shock it climbs to 139 MPa, still safe, but a 25 mm pin would have hit 200 MPa and started bending. If you measure premature pin wear or feel slop in the joint after a season, the most likely causes are: (1) the bushing grease passage is blocked and the joint ran dry — check that the cross-drilled pin Zerk pushes grease through to the joint face, (2) the bracket ears are not coplanar within 0.1 mm so the pin is binding and gall-marking on one side, or (3) the yoke ear has dished outward because the bearing stress σb exceeded 80 MPa, indicating the ear thickness was undersized below 0.5 × pin diameter.
When to Use a Half-yoke Connection and When Not To
The half-yoke is one of three common cylinder rod-end attachments. Each handles side loading, misalignment, and load capacity differently, and the right choice depends on whether the driven member moves in one plane or multiple planes, and how much you can spend on the joint hardware.
| Property | Half-yoke (single-ear clevis) | Spherical rod-eye (self-aligning) | Threaded rod end (rigid) |
|---|---|---|---|
| Degrees of rotational freedom | 1 (planar pivot only) | 3 (full ball joint) | 0 (rigid) |
| Load capacity per unit hardware mass | High — pin in double shear | Medium — limited by ball seat area | Highest — pure axial path |
| Tolerance for misalignment | ±0.5° in one plane only | ±10-15° in any plane | Zero — binds rod immediately |
| Typical cost (50 mm bore cylinder) | £40-80 yoke + pin | £90-180 spherical eye | £15-25 threaded end |
| Typical service life | 5,000-15,000 hours with greasing | 3,000-8,000 hours (ball wears faster) | 20,000+ hours (no wear surfaces) |
| Field repairability | High — replace pin and bushings | Low — replace whole eye | N/A — no wear parts |
| Best application fit | Excavator booms, dump rams, planar arcs | Steering cylinders, multi-axis linkages | Press rams, fixed-line clamps |
Frequently Asked Questions About Half-yoke Connection
This is almost always bracket ear coplanarity. The two ears of the receiving bracket need to be parallel within about 0.1 mm across the pin span — if one ear sits 0.3 mm proud, the pin enters the bushings cocked, and all the load transfers through one edge of one bushing instead of the full bearing area.
Diagnose it by removing the pin and laying a ground straight-edge across both bracket bushings. Any rocking means the ears need re-machining, shimming, or in welded brackets, stress-relieving and re-boring on a line borer. Field fix: a thin bronze shim behind the worn-side bushing buys you time, but the proper fix is line-boring both ears in one set-up.
Map out the actual motion. If the platform pivots on a fixed cross-shaft and the cylinder lies in the same plane as that pivot, a half-yoke is the right answer — it is cheaper, stronger, and rebuildable. If the platform can twist, rack, or the mounting frame flexes under load (common on truck-mounted tippers where the chassis bends), the spherical rod-eye absorbs that out-of-plane motion and saves the rod from side loading.
Rule of thumb: if the frame stiffness gives you more than 1° of out-of-plane misalignment under full load, go spherical. Below that, half-yoke wins on cost and longevity.
You are likely checking shear but missing pin bending. Shear stress assumes the load transfers cleanly across the bushing faces with zero gap. In reality, if the yoke ear is thinner than the bracket ears, or if there is axial play between yoke and bracket, the pin sees a bending moment as well as shear.
Check that the unsupported pin span between the inner faces of the bracket bushings is no more than about 1.2 × pin diameter. Above that, calculate pin bending stress σb = M × c / I and compare against yield, not just shear. A 30 mm pin with a 50 mm unsupported span fails in bending long before it fails in shear.
Only on low-cycle low-load joints. The problem is wear distribution: a hardened pin running directly in a softer steel yoke bore will wear the yoke, not the pin. When the yoke wears, you have to replace or re-machine the yoke — which on a welded cylinder rod end means scrapping the whole rod.
Bushings reverse this — they are sacrificial, cheap, and pressed in. For anything cycling more than a few hundred times a day, bushings pay for themselves the first time you have to service the joint. The only legitimate exception is single-use shock loads like a press tool ejector running a few hundred cycles a year.
Differential thermal expansion. If the pin material has a higher expansion coefficient than the bushings, or if the bushings are insulated from the pin by a grease film and the pin heats faster from internal friction, the running clearance changes by 0.05-0.1 mm over a 40°C rise. That is enough to take a snug joint into audible-knock territory.
The fix is matching materials and giving the joint a clearance specification at operating temperature, not at room temperature. ISO 6020 cylinder builders typically spec bushing-to-pin clearance at 60°C oil temperature for exactly this reason. Also check that grease has not been displaced — fresh grease damps the knock; aged or washed-out grease lets you hear every clearance.
Threaded end-cap with a tab washer beats a cotter pin every time on vibration duty. Cotters back out under sustained vibration — every truck mechanic has seen a tipping ram pin walking out of its bore. Snap rings are fine for clean indoor duty but pop out under shock or contamination.
The threaded-cap and tab-washer arrangement, like Caterpillar uses on excavator pivot pins, locks the pin against rotation and uses a deformable tab to lock the cap thread. Disassembly takes longer but the joint stays put for 10,000+ hours of vibration.
References & Further Reading
- Wikipedia contributors. Clevis fastener. Wikipedia
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