A Variable Eccentric is a rotating drive element where the offset between the input shaft axis and the output crank pin can be adjusted — either while stationary or under load — to change the stroke length of a follower without altering the input speed. Vibratory screening, sand compaction, and press feed equipment all rely on it. The mechanism works by sliding or rotating an inner sleeve relative to an outer hub, which shifts the effective crank radius. The outcome is a single drive that delivers anywhere from zero stroke to full design stroke, replacing what used to require swapping cranks or rebuilding the drive train.
Variable Eccentric Interactive Calculator
Vary RPM, eccentricity, and follower mass to see stroke, peak acceleration, and the centrifugal lock load.
Equation Used
The eccentricity sets the peak radius of the rotating mass, so the peak-to-peak follower stroke is twice the eccentricity. The lock must resist the centrifugal inertial load F from the follower mass rotating at angular speed omega = 2*pi*N/60.
- Eccentricity is the crank radius from shaft center to output pin.
- Force shown is the peak rotating inertial load on the lock.
- Follower mass is treated as rigid and concentrated at the eccentric.
- Friction, damping, bearing losses, and structural compliance are neglected.
Operating Principle of the Variable Eccentric
A Variable Eccentric uses two concentric or near-concentric bodies — typically an outer hub keyed to the drive shaft, and an inner sleeve that carries the output pin. When you rotate or slide the inner sleeve relative to the outer hub, the pin's distance from the shaft centreline changes. That distance is the eccentricity, and it sets the stroke. Double the eccentricity, double the peak-to-peak displacement at the follower. The adjustment can be manual via a locking screw, hydraulic via an internal piston, or automatic via centrifugal counterweights that shift radius with RPM.
The geometry has to be right or you pay for it in vibration. If the inner sleeve sits with even 0.1 mm of slop in the adjustment slot, the eccentric will rattle audibly at speed and the bearing on the output pin will pit within weeks. We hold the sleeve-to-hub fit at H7/g6 — running clearance under 25 µm on a 50 mm bore. The locking mechanism, whether a tapered cone or a wedge clamp, must preload the sleeve against the hub with enough force that the inertial loads at full RPM cannot back-drive the adjustment. On a 1500 RPM screen drive with a 40 mm eccentric and a 30 kg follower mass, the centrifugal load on that lock can hit 30 kN. Undersize the clamp and the eccentricity drifts during operation — you'll see the screen amplitude wander, and the product separation goes off-spec.
Failure modes cluster into three categories. Lock slip is the most common — operator forgets to torque the adjustment screw, and the eccentricity creeps toward zero under vibration. Bearing wear on the output pin is next, usually from running at amplitudes the bearing was never sized for. Third is fretting between the sleeve and hub when the drive runs at near-zero eccentricity for long periods, because the contact patch never moves and microvibration gradually welds and unwelds the surfaces.
Key Components
- Outer Hub: The hub bolts or keys directly to the input shaft and carries the inner sleeve. We typically machine it from 4140 prehard with a bore tolerance of H7 — a 50 mm hub bore should measure 50.000 to 50.025 mm. Anything looser and the sleeve walks under load.
- Inner Adjustable Sleeve: The sleeve carries the output crank pin offset from its own centreline. Rotating the sleeve inside the hub changes the net offset of the pin relative to the shaft axis, from 0 mm (when the two offsets cancel) to twice the sleeve eccentricity (when they align). The sleeve OD must match the hub bore at g6 — running clearance of 9 to 25 µm on a 50 mm size.
- Locking Mechanism: Holds the sleeve angular position against operating loads. Common types are tapered split collars, wedge clamps, or hydraulic expansion locks. The clamp must resist the centrifugal moment at full RPM with at least a 2× safety factor — for a 1500 RPM, 40 mm eccentric on 30 kg follower mass, design for 60 kN minimum.
- Output Crank Pin: Transmits the eccentric motion to the connecting rod or follower. Surface hardness Rc 58-62, ground to h6 to fit the connecting rod bearing. On vibratory service, expect to replace this pin every 8000-12000 hours of full-amplitude operation.
- Adjustment Indicator or Scale: Engraved scale on the hub face that reads sleeve angle versus eccentricity. Resolution should be 1° or better, because at small eccentricities a 5° angular error translates to a 10-15% stroke error. The Schenck and General Kinematics screen drives both use direct angle scales rather than stroke scales because angle reads more linearly.
Real-World Applications of the Variable Eccentric
Anywhere you need to change stroke without stopping production or rebuilding the drive, a Variable Eccentric earns its keep. The classic use case is vibratory screening — you tune amplitude to match material moisture or particle size mid-run. But it shows up in a lot of other places too: press feeders, sand reclamation tables, ore concentrators, and even some dynamic balancing rigs. The economics are simple — one eccentric drive replaces the inventory of fixed-stroke cranks you'd otherwise need.
- Mineral Processing: Schenck Process LinaClass screens use variable eccentric drives so operators can tune amplitude from 4 mm to 8 mm peak-to-peak depending on ore feed grade.
- Foundry Equipment: General Kinematics two-mass vibratory conveyors at foundries like Waupaca Foundry use variable eccentric exciters to dial in conveying speed for different sand grades.
- Construction Equipment: Wacker Neuson reversible vibratory plate compactors employ a centrifugally-actuated variable eccentric that flips polarity for forward and reverse travel without a separate clutch.
- Press Feed Lines: Coe Press Equipment roll feeds use a variable eccentric on the indexing cam to adjust feed length per stroke without changing tooling.
- Concrete Production: Liebherr concrete mixer trucks fit variable eccentric vibrators on the discharge chute to adjust slump-dependent flow.
- Recycling and Aggregate: Metso MF series multi-flo screens at quarry installations use variable eccentric counterweights for amplitude tuning between 6 mm and 12 mm.
The Formula Behind the Variable Eccentric
The core calculation is the net eccentricity as a function of sleeve angle. At the low end of the typical adjustment range — sleeves opposed at 180° — net eccentricity is near zero and the follower barely moves. At the high end — sleeves aligned at 0° — you get twice the sleeve offset and full design stroke. The sweet spot for most screen and feeder applications sits between 60° and 120° because that range gives you the most amplitude change per degree of sleeve rotation, which makes fine tuning easier. Outside that band the response flattens and you lose adjustment resolution.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| enet | Net eccentricity — distance from input shaft axis to output crank pin, equal to half the peak-to-peak stroke | mm | in |
| esleeve | Built-in eccentricity of each element (hub bore offset and sleeve pin offset, assumed equal) | mm | in |
| θ | Angular position of inner sleeve relative to outer hub | degrees | degrees |
| Fc | Centrifugal force on the eccentric mass at the operating speed | N | lbf |
Worked Example: Variable Eccentric in a peat-pellet vibratory screener
Tuning the amplitude on a Variable Eccentric exciter for a 1.8 m wide peat-pellet screening deck at a horticultural processing plant in Vilnius Lithuania, driven by a 7.5 kW motor at 1180 RPM through a 1:1 belt. The exciter has two eccentric elements each with esleeve = 5 mm and a combined eccentric mass of 22 kg. The operator wants to compare stroke at three sleeve settings to match three product grades.
Given
- esleeve = 5 mm
- mecc = 22 kg
- N = 1180 RPM
- θnom = 60 degrees
Solution
Step 1 — at the nominal sleeve angle of 60°, calculate net eccentricity:
Peak-to-peak stroke is 2 × enom = 17.3 mm. That is the working amplitude for medium-grade peat pellets — enough throw to keep the bed fluidised without bouncing fines off the deck.
Step 2 — at the low end of the typical adjustment range, θ = 150° (sleeves nearly opposed):
Peak-to-peak stroke drops to 5.2 mm. At this setting the deck barely shimmies — the operator uses this for fragile dried fines where high amplitude would shatter pellets. Below 2 mm peak-to-peak the bed actually stalls and material packs on the deck.
Step 3 — at the high end, θ = 0° (sleeves fully aligned):
Peak-to-peak stroke is 20 mm. The screen is at maximum throw — used for wet, sticky raw peat that needs aggressive shaking to release moisture. Run too long at this setting and the side-plate weld stress climbs steeply because peak acceleration scales linearly with stroke at fixed RPM.
Step 4 — verify the centrifugal force on the locking mechanism at the high setting:
About 3.4 kN of radial force trying to back-drive the lock. The clamp design needs to hold that with margin.
Result
Nominal net eccentricity at 60° sleeve angle is 8. 66 mm, giving 17.3 mm peak-to-peak stroke. That feels like a firm, controlled shimmy on the deck — enough to fluidise the bed without throwing pellets clear of the screen. Across the operating range, stroke runs from 5.2 mm at θ=150° (gentle, for fragile fines) up to 20 mm at θ=0° (aggressive, for wet raw peat), with the sweet spot for general production sitting between 50° and 80°. If the measured deck amplitude reads lower than predicted, the three usual suspects are: (1) lock screw torque below spec letting the sleeve back off toward zero offset under vibration, (2) rubber isolator mounts under the screen body absorbing throw — check for cracked Rosta or LORD mounts, and (3) eccentric mass loss from a missing balance plug or worn counterweight bolt, which directly reduces the unbalanced moment and starves the system of exciting force.
Choosing the Variable Eccentric: Pros and Cons
A Variable Eccentric isn't always the right tool. Sometimes a fixed eccentric with a swappable counterweight is cheaper and more reliable. Sometimes a Scotch yoke gives you better motion control but no easy stroke adjustment. Here's how the three stack up on the dimensions a screen or feeder designer actually compares.
| Property | Variable Eccentric | Fixed Eccentric with Swappable Weights | Scotch Yoke (Fixed Throw) |
|---|---|---|---|
| Stroke adjustment time | Under 5 minutes, machine running or stopped | 30-90 minutes, machine must be locked out | Not adjustable — requires part swap |
| Stroke range (typical) | 0 to 100% of design max | 3-5 discrete settings | Single fixed value |
| Cost relative to fixed | 1.8× to 2.5× | 1.0× (baseline) | 1.2× to 1.5× |
| Bearing life at full stroke | 8000-12000 hours | 12000-18000 hours | 15000-20000 hours |
| Adjustment lock failure mode | Stroke drift under vibration | N/A — bolted weights | N/A — rigid coupling |
| Best fit application | Multi-product screens, variable feedstock | Single-product high-throughput | Precise sinusoidal motion duties |
| Mechanical complexity | High — moving lock, two concentric bodies | Low — single rigid hub | Medium — sliding yoke and slot |
Frequently Asked Questions About Variable Eccentric
This is almost always lock slip caused by insufficient clamp preload, not by anything wrong with the eccentric geometry itself. The centrifugal force on the inner sleeve creates a small angular moment at every revolution. If the locking torque is below the resisting torque required to hold position, the sleeve creeps toward 180° (zero eccentricity) one fraction of a degree at a time.
Diagnostic check: mark the sleeve and hub with a paint line before startup, then check after 2 hours of running. If the line has shifted, your clamp is the problem. Fix is either retorquing to spec (most makers call for 80-120 Nm on the lock screw) or replacing the lock cone if it's worn smooth.
They solve different problems. A VFD changes RPM, which changes both amplitude (slightly, via centrifugal effects in unbalanced designs) and frequency. A Variable Eccentric changes stroke independently of frequency. For screening, frequency and amplitude have separate optimum points for separation efficiency — high frequency moves fines, high amplitude moves wet sticky stuff.
Rule of thumb: if your product mix needs different stroke values but the same operating frequency, choose Variable Eccentric. If your product mix needs different g-forces but stroke isn't critical, a VFD on a fixed eccentric is cheaper and more reliable.
The cosine relationship isn't linear with angle. At θ = 90°, cos(45°) = 0.707, so net eccentricity is 0.707 × 2 × esleeve, which is actually 70.7% of maximum, not 50%. A lot of operators assume linear behaviour and get caught by this.
If you actually want 50% stroke, set θ such that cos(θ/2) = 0.5, which means θ/2 = 60°, so θ = 120°. Print a stroke-vs-angle table and tape it to the machine — it saves a lot of confusion on the shop floor.
Don't. At zero or near-zero eccentricity, the contact zone between the sleeve and hub never sees any motion relative to the centrifugal load vector, which means the lubricant film never refreshes at that contact patch. Over hours, you get fretting corrosion — micro-welds form and break, leaving rust-coloured debris and pitted surfaces.
If you need to idle, either stop the drive entirely or run at minimum 20% stroke. Schenck specifically warns against zero-eccentricity idling in their manuals for the same reason.
Two likely causes. First, the variable design has more rotating mass concentrated at radius — the sleeve, the lock hardware, the pin assembly. If your foundation isn't designed for the higher unbalanced moment at full stroke, you'll see worse base vibration even at matched stroke.
Second, manufacturing tolerance stacks. A variable eccentric has two offsets that must combine geometrically; small angular errors in the sleeve indexing scale create small but real residual unbalance even when the operator thinks the unit is set to a balanced condition. Check the deck acceleration with a vibration meter at multiple sleeve angles — if you see 5-10% variation that doesn't track the cosine prediction, the indexing scale is miscalibrated and the unit needs to go back for re-zeroing.
2× minimum, 3× preferred. The reason isn't the steady-state load — that's predictable. It's transient events: motor starts, motor stops with material on the deck, emergency stops, and material surge events all create asymmetric loading that briefly spikes the moment on the lock by 50-150% above steady-state.
For our worked example with 3.4 kN steady-state radial load, design the clamp for at least 7 kN holding capacity. Most commercial Variable Eccentric makers, including General Kinematics and Metso, design to 3× routinely.
References & Further Reading
- Wikipedia contributors. Eccentric (mechanism). Wikipedia
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