Crank Equalizing Angle is the angular spacing between throws on a multi-cylinder reciprocating air compressor crankshaft, set so each piston's compression stroke peaks at evenly-spaced points in one revolution. The mechanism distributes torque demand across the rotation so the motor and flywheel see steadier load instead of one big spike per cycle. It exists to cut torque ripple, vibration, and pulsation in the discharge line. A correctly equalized 2-cylinder pump runs at 180° spacing, and a 3-cylinder W-pattern at 120° — the difference between a smooth Ingersoll Rand 2475 and a compressor that walks across the floor.
How the Crank Equalizing Angle Works
The Crank Equalizing Angle, also called the Crank Equalizing Angle in Air Compression when you want to be unambiguous in a pneumatic context, is a phasing decision baked into the crankshaft geometry the day it gets forged. Each cylinder of a reciprocating compressor demands peak torque near the top of its compression stroke — typically the last 30° before TDC. If you stack two cylinders on the same throw, both pistons hit peak load at the same instant, the motor sags, and the discharge pulses. Space the throws evenly around 360° and each torque peak gets its own slice of the rotation, so the flywheel only has to bridge small gaps instead of one massive valley.
The rule is simple: equalizing angle = 360° / number of cylinders, for single-acting pumps. Two cylinders go 180° apart. Three cylinders sit at 120°. A four-cylinder boxer compressor pairs 0°/180° and 90°/270°. Get this wrong by more than about 3° and you'll feel it — the compressor base hops on its isolators, the head bolts on the loaded cylinder loosen faster, and the pressure ripple at the tank inlet jumps from a clean 2-3 psi to 8 psi or more. The check valve at the tank then chatters and wears out in months instead of years.
Why not just put a bigger flywheel on a poorly-phased pump? You can, and small benchtop compressors do exactly that. But flywheel inertia scales with mass and radius squared, so doubling the smoothing effect quadruples the rotating mass — bad for startup current, bad for bearing life, bad for the operator who has to lift the head off. Proper crank phasing solves the problem at the source for almost no added cost.
Key Components
- Crankshaft Throws: The offset journals on the crankshaft that drive each connecting rod. Their angular spacing around the shaft axis IS the equalizing angle. Tolerance on the index angle is typically ±0.5° on a quality forged crank like those in the Quincy QR-25 series — anything looser and the torque peaks drift out of equalization.
- Connecting Rods: Transfer piston force to the throws. Rod length-to-stroke ratio affects when peak torque actually occurs relative to TDC — a short rod (ratio under 3.5) shifts peak torque later in the stroke, which can blunt the benefit of textbook equalizing angles.
- Flywheel: Stores kinetic energy between torque peaks. With proper equalizing, flywheel mass moment of inertia can be 30-50% lower than a single-cylinder equivalent for the same speed regulation (typically Cf = 0.02 for compressors). Cast iron flywheels of 15-30 kg are common on 5-10 HP pumps.
- Counterweights: Forged into the crank webs opposite each throw to cancel rotating-mass imbalance. Equalizing angle handles torque smoothness; counterweights handle shaking force. Both are needed — get either wrong and the compressor vibrates.
- Discharge Manifold: Combines the flow pulses from each cylinder. With correct equalizing, the manifold sees overlapping discharge events that smooth the flow before it reaches the tank, dropping pressure pulsation by typically 60-70% versus a single-cylinder pump of equivalent CFM.
Who Uses the Crank Equalizing Angle
Crank Equalizing Angle in Air Compression shows up everywhere reciprocating pumps are still the right tool — and despite the rise of rotary screw machines, that's a much wider list than people assume. Anywhere torque ripple, vibration, or pulsation matters, the equalizing angle is doing quiet work in the background.
- Industrial Shop Air: Ingersoll Rand 2475N7.5 two-stage compressors use a 180° equalized V-twin crank to run smoothly at 1,000 RPM pump speed feeding 175 psi shop air.
- Locomotive Air Brake Systems: Westinghouse 3-CD3 cross-compound air pumps on legacy diesel-electric locos use a duplex equalized crank to charge the train brake pipe without lugging the auxiliary generator.
- Dental & Medical Air: Powerex SED series oilless reciprocating compressors run 3-head 120° equalized cranks specifically to keep pressure pulsation under 1.5 psi peak-to-peak for hand-piece stability.
- Marine Starting Air: Sperre HV2/200 marine compressors use a vertical 2-cylinder 180° equalized layout to charge starting air receivers to 30 bar on auxiliary engines.
- PET Bottle Blowing: High-pressure 40 bar PET booster compressors like the Bauer PE-VE series use 3-cylinder W-block equalized cranks to deliver pulsation-free air to the blow-mold valve trains.
- Mine Rescue & Breathing Air: Bauer K14 four-stage breathing air compressors stagger their four pistons at 90° equalizing angles to keep the inter-stage cooler flow steady at 225 bar final discharge.
The Formula Behind the Crank Equalizing Angle
The basic equalizing angle is straightforward arithmetic, but the practical question is how torque ripple changes as you move across the operating range. At the low end of typical shop-compressor speeds — say 600 RPM — the flywheel has more time per revolution to smooth gaps, so equalizing errors of 2-3° are tolerable. At nominal 1,000-1,200 RPM the equalization needs to be tight or you'll feel the beat in the baseplate. Push to 1,750 RPM on a directly-coupled pump and even 1° of phasing error produces measurable vibration at the motor mounts. The sweet spot for most cast-iron industrial pumps sits around 800-1,200 RPM where bearing life, valve life, and pulsation all line up.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| θeq | Crank equalizing angle between adjacent throws | degrees | degrees |
| ncyl | Number of single-acting cylinders sharing the crank | count | count |
| Tpeak | Peak compression-stroke torque per cylinder | N·m | lb·ft |
| Tripple | Residual torque variation seen by the motor shaft | N·m | lb·ft |
Crank Equalizing Angle Interactive Calculator
Vary cylinder count and crank throw spacing to see the ideal equalizing angle, throw positions, and torque ripple pattern.
Equation Used
The calculator uses the article rule for a single-acting compressor: divide one revolution by the number of cylinders to get the ideal crank equalizing angle. It then compares the entered actual spacing to that ideal and lays out the resulting throw positions.
- Single-acting reciprocating compressor
- Throws are intended to be evenly spaced over one revolution
- Spacing error is the absolute difference between actual spacing and ideal theta
Worked Example: Crank Equalizing Angle in a textile mill plant air compressor
A cotton spinning mill in Ahmedabad runs a 3-cylinder W-block reciprocating air compressor — Kirloskar KC3 frame — to feed 7 bar plant air to ring-frame pneumafil suction cleaners. You need to verify the equalizing angle and estimate torque ripple at three operating speeds: 600 RPM (idle/unload), 950 RPM (nominal duty), and 1,400 RPM (peak demand). Peak per-cylinder compression torque is measured at 145 N·m at 7 bar discharge.
Given
- ncyl = 3 cylinders
- Tpeak = 145 N·m
- RPM range = 600 / 950 / 1400 RPM
Solution
Step 1 — compute the ideal equalizing angle from cylinder count:
So throws are indexed at 0°, 120°, and 240° around the crank. The Kirloskar KC3 forged crank holds this to ±0.4° — well inside the ±0.5° tolerance band where torque smoothing stays effective.
Step 2 — estimate residual torque ripple at nominal 950 RPM, where the equalizing angle does its main job:
This is the torque variation the motor and flywheel must absorb between firing events. On a 7.5 kW motor this represents roughly 0.5× the mean torque — manageable with a 22 kg cast-iron flywheel running Cf = 0.02 speed regulation.
Step 3 — at the low end of the operating range, 600 RPM during unloaded periods:
At 600 RPM the rotation period is 100 ms versus 63 ms at nominal, so the flywheel kinetic energy buffer is more effective and the operator sees almost no perceptible vibration at the baseplate — readings under 2 mm/s RMS on a vibration meter.
Step 4 — at the high end, 1,400 RPM peak demand:
The torque pulses arrive every 14.3 ms. The 22 kg flywheel can still bridge them, but valve float starts at this speed on standard reed valves and pulsation at the tank inlet climbs from 2.8 psi peak-to-peak at nominal up to 6 psi at 1,400 RPM. Above this you should not run a Kirloskar KC3 continuously regardless of equalizing — it is a duty-cycle limit, not an equalizing limit.
Result
Nominal equalizing angle is 120° with a residual torque ripple of 72. 5 N·m at 950 RPM. That is the load swing the motor sees between cylinder firings — small enough that a properly sized 22 kg flywheel keeps speed regulation under 2% and baseplate vibration around 3 mm/s RMS. Across the range the geometry stays the same but the felt experience changes: at 600 RPM the pump is dead smooth, at 950 RPM you feel a faint pulse through your boots, and at 1,400 RPM the tank inlet pulsation doubles and reed valves begin to float. If your measured ripple exceeds 90 N·m at nominal, suspect one of three causes: (1) one cylinder's intake valve leaking back so its compression stroke makes less torque and breaks the equalization, (2) a sheared crank dowel letting one throw slip out of index by 5°+, or (3) a cracked flywheel hub allowing micro-slip on the taper, which doesn't change geometry but kills the energy buffer.
Crank Equalizing Angle vs Alternatives
Equalized multi-cylinder cranks aren't the only way to get smooth air. Single-cylinder pumps with oversized flywheels and rotary screw compressors both compete in the same CFM bands. The right pick depends on duty cycle, noise budget, and how much vibration the building can absorb.
| Property | Equalized 3-cyl Reciprocating | Single-cyl Reciprocating + Heavy Flywheel | Rotary Screw |
|---|---|---|---|
| Typical speed range (RPM) | 600-1,400 | 400-800 | 1,800-3,600 |
| Torque ripple (% of mean) | 40-50% | 120-180% | 5-10% |
| Discharge pulsation (psi p-p) | 2-6 | 8-15 | <0.5 |
| Capital cost (5 HP class) | $1,200-2,000 | $600-1,000 | $3,500-6,000 |
| Maintenance interval (hrs) | 2,000 valve service | 1,500 valve service | 8,000 oil/filter |
| Service life to overhaul (hrs) | 15,000-25,000 | 8,000-12,000 | 40,000-60,000 |
| Best duty cycle | Up to 75% | Up to 50% | 100% continuous |
| Typical CFM at 100 psi | 15-40 | 5-15 | 20-200+ |
Frequently Asked Questions About Crank Equalizing Angle
180° phasing equalizes torque, not necessarily flow. On a single-acting 2-cylinder pump both pistons discharge once per revolution but each cylinder only delivers air during roughly the last 60° of compression. That means you get two ~60° flow pulses with ~120° gaps between them — the discharge stream is still highly intermittent.
Fix is downstream: add a pulsation dampener bottle of about 10× the per-stroke displacement directly at the manifold outlet, or run a longer feed line to the tank to let the volume buffer the pulses. The crank is doing its job; the air column needs help.
Almost never worth it. The bottom-end bearing loads, oil pump capacity, and crankcase breathing on a 2-cylinder frame are sized for two firing events per revolution. Doubling the cylinders doubles the heat rejection, halves the bearing rest periods, and usually cracks the frame webs within a few thousand hours.
If you need 4-cylinder smoothness, buy a frame designed for it — Quincy QT-54 or Ingersoll Rand 7100 class. The retrofit math always loses to a purpose-built block.
±0.5° is the working tolerance for industrial pumps up to 1,200 RPM. Above 1,500 RPM tighten to ±0.25° or you'll hear a beat frequency at the baseplate.
Regrinders sometimes hit this with a rotary index fixture but more often miss because they index off a worn keyway. Always index off the original throw centerline — never off a keyway that's been hammered on for 20 years. A drift of 2-3° from keyway wear is common and it'll wreck your equalization.
The 3-cylinder pays off when duty cycle is above 60% and the building or process is sensitive to vibration — laboratories, calibration rooms, anywhere with sensitive measurement equipment within 10 m of the compressor. Torque ripple drops from roughly 100% of mean torque on a 2-cylinder to about 50% on a 3-cylinder.
For typical 30% duty shop air feeding impact wrenches and blow guns, the 2-cylinder is fine and saves you 25-30% on capital cost. Don't pay for smoothness you don't need.
A correctly equalized 3-cylinder pump shows a clean 3× running-speed peak on the FFT — at 950 RPM that's 47.5 Hz dominant. Phasing error of 5° or more produces a strong 1× running-speed component (15.8 Hz) on top, because one cylinder's torque pulse is no longer cancelled by its neighbours' spacing.
Rule of thumb: if the 1× peak is more than 30% of the 3× peak amplitude, you have either a phasing problem, a leaking valve on one cylinder, or an unbalanced flywheel. Pull the head on the suspect cylinder first — it's the cheapest check.
Yes, and people get this wrong constantly. A double-acting cylinder fires twice per revolution — once on each end of the piston — so it counts as two events. A 2-cylinder double-acting compressor has 4 firing events per rev, so the equalizing angle is 90°, not 180°. Throws are still 180° apart on the crank, but the head end and crank end of each cylinder fire at offset crank positions, giving you 90° equalization on the torque trace.
This is why old Ingersoll Rand PHE class double-acting pumps run so smoothly at 300-450 RPM — they get 4-event smoothness from 2 cylinders.
References & Further Reading
- Wikipedia contributors. Reciprocating compressor. Wikipedia
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