A Yoke Strap is a twin-legged motion-constraining fitting — two parallel arms joined at a common head — that wraps around a shaft, rudder post, or load and transfers force symmetrically to a single output point. You see it on outboard tiller arms, sailboat rudder heads, and balanced lifting slings. The two legs share the load, cancel side bending, and keep the constrained part rotating or translating on a clean axis. The result is reduced shaft fatigue and predictable motion under cyclic load.
Yoke Strap Interactive Calculator
Vary input force and included yoke angle to see equal leg tension, spread load, and cancelled side load.
Equation Used
The symmetric yoke strap splits the input force F into two equal leg tensions T. The forward components add to the applied force, while the lateral components cancel; increasing the included angle theta increases leg tension and spreading load.
- Two equal-length yoke legs share the load symmetrically.
- Pins and legs are ideal, with no friction or slack.
- Net side load is zero only for perfect left-right symmetry.
- Default force is normalized to 1 N to reproduce the worked example T/F ratio.
Inside the Yoke Strap
A Yoke Strap works by splitting one input force across two symmetrical legs that both terminate on the same axis as the constrained part. Picture an outboard motor steering yoke — the tiller pulls the head of the strap, both legs swing the rudder shaft together, and the shaft sees pure torque instead of torque plus a side load. That symmetry is the whole point. If you tried to drive the same shaft with a single arm clamped to one side, the bearing on that side would carry a bending moment and wear out in a fraction of the cycles.
Geometry matters. The two legs must be the same length within roughly 0.5 mm on small marine hardware, and the included angle between them — measured at the head — sets how much of the input force becomes useful torque versus how much becomes leg tension. Tighter angles (under 30°) make the legs work hard in tension and amplify any pin slop; wider angles (over 90°) waste input travel. Most production yoke straps land between 40° and 70° because that's where you get clean torque transfer without the legs trying to spread under load.
Failure modes are predictable. If the clevis pins at the leg ends develop more than about 0.2 mm of radial play, the strap starts to walk under reversing load and you'll hear a knock at each direction change. If one leg is even slightly shorter than the other — manufacturing error, or a repair weld that pulled the metal — the shaft sees an asymmetric pull and the upper bearing wears oval. Hot-dip galvanised straps used in marine service can also lose section at the head where stress concentrates, and that's where 90% of fatigue cracks initiate.
Key Components
- Head (yoke crown): The single connection point where the input force enters. Usually drilled for a clevis pin or shackle. The head must be thicker than the legs — typically 1.5× leg thickness — because stress concentrates here. A sharp internal radius below 3 mm at the leg-to-head transition is the classic crack initiation site.
- Legs (arms): Two parallel or angled arms that carry the load symmetrically to the constrained part. Length tolerance is critical — 0.5 mm mismatch on a 200 mm leg pair will tilt the constrained shaft enough to overload one bearing. Cross-section is sized for tension plus a 1.5× shock factor in marine service.
- Clevis ends: The terminations at the bottom of each leg, drilled for pins that connect to the rudder post lugs, tiller arms, or sling shackle. Pin holes must be reamed not drilled, with bore tolerance H8 or tighter. More than 0.2 mm radial pin play causes the audible knock under reversing load.
- Spreader (optional): On long straps — typically over 300 mm leg length — a transverse spreader bar locks the legs in a fixed angle so they can't pinch or splay under cyclic load. Skipped on small fittings; mandatory on lifting yokes rated above 1 t.
- Pins and shackles: Connect the strap to the load and to the input. Shoulder pins with cotter retainers are standard on marine hardware. For lifting service, only forged shackles with proven WLL stamps are acceptable — no welded substitutes.
Real-World Applications of the Yoke Strap
Yoke Straps appear anywhere you need to apply rotation, traction, or lift to a part without introducing side load. They're cheap, passive, and have no moving parts beyond the end pins, which is why they survive on equipment that gets wet, dirty, or shock-loaded. You'll find them at small scale on tiller hardware and at large scale on shipyard lifting rigs.
- Marine hardware: Tiller yoke on Yamaha and Mercury outboard remote-steering kits, where the strap couples the cable pull to the engine's steering arm without bending the steering shaft.
- Sailing: Rudder head yoke on Laser and Sunfish dinghies — the steering ropes pull the strap head, both legs swing the rudder stock evenly.
- Lifting and rigging: Two-leg balanced sling on Crosby and Gunnebo lifting yokes used to upend wind turbine tower sections at fabrication yards like CS Wind in Vietnam.
- Aerospace ground support: Engine-mount strap on jet engine transport cradles — the yoke distributes shock loads from forklift handling across both engine pickup points instead of one.
- Heavy haulage: Recovery yoke straps on Miller Industries wreckers, where the strap clears the towed vehicle's frame rails and pulls evenly on both lifting eyes.
- Industrial doors: Counterweight yoke on overhead crane bridge doors and large rolling shutters — the strap transfers cable pull to a horizontal shaft without inducing bow.
The Formula Behind the Yoke Strap
Sizing a Yoke Strap comes down to one calculation: the tension in each leg as a function of the input force and the included angle between the legs. At small included angles the legs run almost parallel and each carries roughly half the input force — that's the cheap, easy regime. As the angle opens up, leg tension climbs fast, and beyond about 90° you're paying a real penalty. The sweet spot for most marine and rigging yokes sits between 40° and 70° included angle. Below 40° you've made the strap unnecessarily long; above 90° you're oversizing the legs to handle tension that geometry created for no reason.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Tleg | Tension carried by each individual leg of the yoke strap | N | lbf |
| Finput | Total input force applied at the head of the yoke | N | lbf |
| θ | Included angle between the two legs, measured at the head | degrees | degrees |
Worked Example: Yoke Strap in a sailboat rudder head yoke
You are sizing the stainless rudder yoke strap on a 9.5 m J/30 keelboat being refit at a yard in Newport Rhode Island. The steering quadrant pulls the yoke head with a peak helm force of 800 N during a hard round-up. The two legs terminate at lugs on the rudder stock head, and you need to know each leg's working tension so you can pick a leg cross-section that won't fatigue over a 10-year service life.
Given
- Finput = 800 N
- θ (nominal) = 60 degrees
- Material = 316 stainless —
Solution
Step 1 — at the nominal included angle of 60°, compute the leg tension. Half of θ is 30°, and cos(30°) = 0.866:
Step 2 — at the low end of the typical operating range, 40° included angle, the legs run more parallel and each carries closer to half the input:
That's only 36 N less than the nominal — a marginal saving for a much longer strap. You're not gaining anything practical by making the strap skinnier and longer.
Step 3 — at the high end, 90° included angle, the legs splay wide:
That's a 22% jump in leg tension over nominal. On a 6 mm × 25 mm 316 stainless leg cross-section the working stress climbs from about 3.1 MPa to 3.8 MPa — still safe statically, but the head fillet stress concentration multiplies that by 3-4×, and at 90° the strap is starting to live closer to the fatigue knee for cyclic helm loads.
Result
Each leg of the J/30 rudder yoke carries 462 N at the nominal 60° included angle under 800 N peak helm force. That's a comfortable working tension for a 6 mm × 25 mm 316 stainless leg — well below the fatigue limit, with margin for shock loads when the boat slams off a wave. Across the operating range, leg tension goes from 426 N at 40° to 566 N at 90°, so the geometry sweet spot is firmly in the 50-70° band where you minimise both leg tension and strap length. If you measure leg strain in service and the reading is 30% higher than predicted, the most likely causes are: (1) one leg shorter than the other by more than 0.5 mm, throwing all the load onto the shorter leg, (2) a clevis pin bore that's been wallowed out past 0.3 mm radial play so the strap is loading on one shoulder only, or (3) a sharp internal radius at the head-to-leg fillet below 3 mm, which spikes local stress and shows up as elevated readings on a strain gauge near the crown.
When to Use a Yoke Strap and When Not To
A Yoke Strap is the right answer when you need symmetric load transfer with no moving parts, but it's not the only way to drive a shaft or lift a load. Here's how it stacks up against the two alternatives you'll most often consider — a single-arm lever and a two-leg chain bridle.
| Property | Yoke Strap | Single-Arm Lever | Two-Leg Chain Bridle |
|---|---|---|---|
| Load symmetry | Symmetric, both legs share load equally if length-matched | Asymmetric, induces bending moment on shaft | Symmetric but flexible, load split depends on chain length match |
| Cost (typical small marine fitting) | $40-120 forged or laser-cut stainless | $15-40 single bar | $60-200 chain plus master link plus shackles |
| Lifespan under cyclic load | 10-20 year service in marine use, fails at head fillet | 3-7 year service, fails at shaft-to-arm joint from bending fatigue | 5-15 years, chain links wear at pin contact |
| Load capacity range | 50 N to 50 kN typical | 10 N to 5 kN typical (bending limited) | 1 kN to 500 kN typical (lifting service) |
| Side-load on driven shaft | Near zero — pure torque | High — bending plus torque | Near zero if rigged correctly |
| Application fit | Rudder yokes, tiller arms, balanced lift fixtures | Quick-and-dirty steering, prototype linkages | Heavy lifting, crane rigging, turbine handling |
| Maintenance interval | Annual pin inspection in marine service | Pin and arm inspection every 6 months | Chain inspection every 3 months in lifting service |
Frequently Asked Questions About Yoke Strap
Length-matched legs only share load evenly if the head pivot and the two clevis pins all stay coplanar. If the rudder stock has any axial play or the head pin is mounted on a flexing bracket, the strap can rock out of plane — one leg goes into tension, the other goes slack, and you'll feel it as a soft spot in the helm.
Check the head bracket stiffness first. On fibreglass-mounted hardware, a backing plate that's deflecting under load is the usual culprit. A 2 mm steel doubler under the bracket usually solves it.
For non-critical service — a tiller yoke on a small dinghy, say — a TIG repair on a clean parent material can work, provided you stress-relieve afterwards and inspect for porosity. The catch is that a weld rarely matches the original leg length within 0.5 mm, and any length mismatch loads the unwelded leg disproportionately.
For lifting service or any rated fitting, replacement is the only legal answer. A welded lifting yoke loses its WLL certification the moment you strike an arc on it.
Start with the geometry your driven part forces on you — the spacing of the rudder stock lugs, or the pickup points on the load. That sets the leg-end separation. Then pick a head height that puts the included angle between 50° and 70°. Below 40° the strap gets unnecessarily tall and the legs barely benefit from the wider geometry. Above 90° leg tension climbs fast — at 120° it's nearly equal to the input force itself.
Rule of thumb: head height ≈ 0.7× to 1.2× the leg-end separation gets you in the sweet spot.
The formula assumes the input force pulls perfectly along the strap's centreline of symmetry. In practice, steering cables and lifting rigs rarely pull exactly on-axis — a 10° off-axis pull at the head adds a cosine penalty that shows up as elevated tension in whichever leg is on the loaded side.
Diagnostic: put a strain gauge on each leg and compare. If the readings differ by more than 5%, the input is pulling off-axis, not centreline. Realign the cable lead or the lifting shackle and the readings should converge.
Fatigue cracks initiate where stress concentrates, not where average stress is highest. The head-to-leg fillet sees a stress concentration factor of 3 to 4 if the internal radius is sharp — under 3 mm. So even though average leg stress might be 50 MPa, peak local stress at the fillet can hit 200 MPa, which is well into the fatigue regime for cyclic helm loads at thousands of cycles per season.
If you're designing a strap from scratch, generous fillet radii at the crown — 5 mm or more — push the fatigue life out by an order of magnitude. It's the single biggest design lever you have.
You can, but only over a limited rotation range — typically ±30° from the strap's neutral position. Beyond that the included angle changes enough that leg tension climbs nonlinearly and the actuator sees a varying load curve. For full 90° or 180° shaft rotation, a rack-and-pinion or a four-bar crank arm gives you more linear behaviour.
For small-angle steering applications — outboard tillers, autopilot rams driving a rudder — a yoke strap paired with a Linear Actuator works well and is what most marine autopilot installs use.
References & Further Reading
- Wikipedia contributors. Yoke. Wikipedia
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