Eccentrics and cranks look similar on a drawing, yet they behave like distant cousins once torque starts flowing. Knowing which one to specify can save an entire power-train from premature failure.
Mislabeling them on a bill of materials invites rework, warranty claims, and field service trips that dwarf the original part cost. This article maps every practical difference so you can select, size, and troubleshoot with confidence.
Core Geometry That Separates the Two
An eccentric is a circle whose center is offset from the shaft axis; the amount of offset is called the throw. A crank is an assembly of two journals—main and rod—connected by a cheek or web, creating a discrete crankpin.
Because the eccentric’s outer race is a continuous cylinder, it can be machined in one chucking on a lathe. The crank’s rod journal must be milled, ground, and often induction-hardened separately, adding setups and cost.
Throw versus stroke: throw equals the offset in an eccentric, while stroke is twice the crankpin offset. A 20 mm eccentric throw yields 40 mm total reciprocating travel, identical to a crank with 20 mm crankpin offset.
Visual Cue Quick Test
Spin the shaft; if the apparent outer diameter wobbles but never changes radius, it is an eccentric. If a secondary pin orbits the main axis, you are looking at a crank.
Load Paths and Force Vectors
Eccentrics transmit radial loads that rotate around the shaft, producing purely sinusoidal motion in the follower. Cranks convert torque into alternating tangential and radial forces on the crankpin, plus a rotating bending moment on the main journal.
The bending moment means crank webs must be sized for fatigue; web thickness often exceeds the journal diameter. Eccentrics avoid this because the load vector passes through the shaft centerline, eliminating bending.
Finite-element studies show peak stress in a crank concentrates at the fillet between journal and web, whereas an eccentric shows uniform hoop stress around the offset ring. Designers can therefore use thinner shafts with eccentrics when bending is the limiting case.
Bearing Life Reality Check
A pump eccentric running at 1 800 rpm with 5 kN radial load can use a standard deep-groove ball bearing. The same load on a crankpin demands a cylindrical roller bearing with 30 % higher dynamic capacity to survive the same design life.
Manufacturing Cost and Lead-Time Drivers
One-piece eccentrics turned from bar stock need only a second operation to mill the keyway. Cranks require forgings or castings, multiple turning setups, and often balancing drills that add 15 % to cycle time.
Material utilization favors eccentrics: a 3 kg eccentric starts as 3.2 kg bar, while a 3 kg crank begins as a 5 kg forging slug. Machining chips alone can double the variable cost for high-volume programs.
Lead-time for a custom eccentric in 4140 steel is typically two weeks; a comparable crank from a closed-die forging can stretch to eight weeks due to die manufacturing and heat-treat queues.
Prototyping Hack
For functional prototypes, shops can weld two offset discs to a shaft to mimic an eccentric, then turn the assembly concentric to the desired throw. Cranks cannot be mocked up this way because welded webs fail under reversed bending.
Balance and Vibration Signatures
Static balance is automatic in a symmetric eccentric mounted between identical counterweights. Cranks need computer-aided balancing to place drill holes at exact angular positions, or else first-order inertia forces shake the frame.
Dynamic balance becomes critical above 3 000 rpm; a crankshaft for a motorcycle engine receives multi-plane correction to within 0.5 g·cm. An eccentric running the same speed may need only a single correction plane because its mass distribution is inherently uniform.
Field data from packaging machinery shows switching from a crank to an eccentric reduced overall vibration velocity from 4.5 mm/s rms to 1.8 mm/s rms, eliminating the need for a tuned-mass damper.
Balance Budget Rule
Allow 2 % of rotating mass for eccentric counterweights, but reserve 5–7 % for crank counterweights to hit ISO 10816 G2.5 limits.
Speed Envelopes and Efficiency Maps
Eccentrics excel below 1 500 rpm where sliding friction dominates; their plain bearings ride on hardened sleeves with micro-honed finishes. Cranks tolerate 15 000 rpm in automotive engines because rolling-element bearings isolate the sliding action.
Mechanical efficiency peaks differently: an eccentric press at 30 strokes per minute converts 92 % of motor torque into slide force thanks to minimal joints. A crank press at 200 spm drops to 85 % due to alternating bearing losses and windage.
Above 20 m/s rubbing velocity, eccentrics require pressurized oil films or switching to a crank configuration becomes mandatory. That threshold often dictates the choice in high-speed textile looms.
Efficiency Snapshot
At 500 rpm and 10 kW, an eccentric drive consumes 0.8 kW in friction losses. A crank doing the same work needs 1.2 kW, a 50 % penalty paid in heat.
Application Sweet Spots
Camshaft timing belts use eccentrics to tension the belt; a simple wrench turn adjusts center distance without adding pivot brackets. Bicycle cranks must be cranks because rider torque exceeds the bending capacity of an equivalent eccentric arm.
Steam locomotive valve gear relied on eccentrics for reverse motion because reversing the engine rotation reversed the valve sequence automatically. Modern compressors favor cranks when multi-throw arrangements are needed to cancel reciprocating mass.
Automotive power steering pumps miniaturized the eccentric concept: a 20 mm throw ring inside the rotor creates five pumping chambers per revolution, delivering 120 bar with whisper-quiet operation.
Selection Matrix
Use eccentrics for single-plane, low-speed, moderate-load motion. Choose cranks when stroke must exceed 100 mm, speed climbs above 3 000 rpm, or multi-cylinder balancing is required.
Maintenance Access and Wear Modes
Eccentric wear shows up as an increase in throw, detectable with a simple dial indicator while the shaft rotates. Crank wear manifests as journal ovality, requiring micrometer measurements at 90° indexes.
Replacing an eccentric takes one puller and ten minutes in a beverage can seamer. Replacing a crank in the same machine demands removing the entire side frame, a four-hour job that production schedules hate.
Lubrication intervals differ: an eccentric with bronze bushings runs 2 000 hours between grease shots. A crankpin roller bearing in the same duty cycle needs fresh oil every 500 hours because contamination migrates along the needle cage.
Predictive Metric
Track eccentric throw growth beyond 0.05 mm to schedule replacement before follower slap appears. For cranks, journal taper above 0.01 mm predicts imminent bearing seizure.
Design Shortcuts and Calculation Aids
Quick stroke check: measure the distance between extreme follower positions; divide by two to get eccentric throw. Crank stroke equals twice the crankpin offset, so a 35 mm stroke demands 17.5 mm offset—no trigonometry required.
Force estimation: eccentric radial load equals follower force divided by cosine of the pressure angle, usually under 15° so the cosine is near unity. Crankpin load is torque divided by crank radius, but peaks at 2.5× average due to angle changes.
Finite-element newbies can model an eccentric as a thick-walled cylinder with offset centroid; convergence needs only 20 000 nodes. A crank model requires hex meshes at fillets with 0.1 mm element size, pushing node count past 200 000.
Excel Formula
For eccentric stroke use =2*OFFSET. For crankpin bearing life use =((C/P)^3.33)*10^6/60 to get hours at constant speed.
Retrofit and Upgrade Strategies
Swapping a crank punch press to eccentric drive eliminates the flywheel, claw clutch, and brake, freeing floor space. The trade-off is a fixed stroke; adjustable stroke now requires a servo motor and ballscrew instead of a simple shim.
Converting the other way—eccentric to crank—happens when line speed doubles. A packaging line upgraded from 60 to 120 packages per minute retrofitted eccentric carton pushers with crank-and-rod mechanisms to avoid overheating.
Always verify frame clearance: an eccentric’s follower housing moves in a circle, demanding a 10 mm radial clearance pocket. A crank’s rod swings in a 180° arc, so check for interference with guards and conduit runs.
Cost Payback
One food plant saved $28 000 yearly in spare parts after replacing six crank assemblies with sealed eccentric units; payback arrived in 14 months despite the higher initial price per unit.
Sound and Noise Control Tactics
Eccentrics generate tonal noise at shaft frequency plus follower slap harmonics. Adding a 0.1 mm pre-load spring eliminates backlash and drops sound pressure by 6 dB(A).
Cranks produce broadband noise from bearing rollers impacting the outer race. Switching to super-finished raceways and using random-pitch rollers spreads the spectrum, making the machine 3 dB(A) quieter to human ears.
Acoustic camera tests reveal that eccentric-driven screeners radiate noise from the follower plate, not the shaft. A simple constrained-layer damping pad glued to the plate cuts 4 dB(A) for under fifty dollars.
Quiet Design Tip
Specify eccentrics with polymer-lined bearings; the compliant layer absorbs 30 % of impact energy, a trick impossible on a hardened crankpin.
Environmental and Sustainability Angles
Energy saved by choosing an eccentric in a 50 kW blower application running 8 000 h/year totals 3 200 kWh, trimming 1.6 t COâ‚‚. The savings come from lower friction and the elimination of a crankcase oil sump.
Material recyclability favors eccentrics: one alloy grade can be remelted straight back to bar stock. Cranks often contain copper-lead bearings that must be removed before steel scrap enters the electric arc furnace.
End-of-life disassembly time for an eccentric averages five minutes with a single wrench. Cranks require bearing pullers, snap-ring pliers, and a press, stretching removal to 30 minutes and discouraging proper recycling.
Carbon Ledger
Over a 20-year life, an eccentric drive adds 0.8 t COâ‚‚e; an equivalent crank drive hits 1.2 t COâ‚‚e, mostly from extra electricity and bearing production.