Three Compressor Gearbox Types — Where Helical Gears Fit
A separate enclosed helical gear gearbox between the motor and the centrifugal compressor, increasing motor speed (1,500 or 3,000 RPM) to impeller speed (5,000–30,000 RPM) in one or two stages. This is the most common configuration for large industrial centrifugal compressors (500 kW–50 MW) and is governed by API 613 Special Purpose Gear Units for Petroleum, Chemical and Gas Industry Services.
The impellers are mounted directly on the extended ends of the high-speed pinion shafts — there is no separate coupling between the gear and compressor. The gear body forms part of the process boundary. Multiple stages of compression can be achieved by mounting impellers on different pinion shaft extensions at different gear ratios, all driven from one central bull gear on the motor input shaft.
Oil-free screw compressors use synchronising timing gears to maintain precise clearance between the male and female rotors without physical contact. The timing helical gears transmit no process load (the rotor contact provides torque transmission in oil-flooded types) but must maintain exact rotor phase relationship — making backlash and pitch accuracy the critical parameters.
Centrifugal Compressor Speed Increaser — Gear Design Requirements
High-speed helical gear for centrifugal compressor speed increaser — the combination of DIN Class 3–5 accuracy, Ra ≤ 0.2 µm tooth flank surface, and low-helix-angle (β = 5–15°) design limits the gear vibration contribution to API 613’s maximum allowable 25 µm peak-to-peak vibration at the bearing housing
Pitch-Line Velocity — The Dominant Design Constraint
The pitch-line velocity v = π × d₁ × n₁ / 60,000 [m/s] for a centrifugal compressor speed increaser pinion ranges from 25 m/s for a slow natural gas compressor to over 200 m/s for a high-speed multistage air compressor. At these velocities:
- Tooth mesh friction loss dominates: At v > 30 m/s, the no-load viscous mesh loss P_VZO ∝ v² becomes a significant fraction of transmitted power. For an API 613 compressor speed increaser helical gear, ISO VG 32–68 turbine oil (not gear oil) is specified — the lowest viscosity grade that maintains adequate EHL film — to minimise churning losses at high speed.
- Dynamic load factor KV becomes significant: At v = 50 m/s for a DIN Class 8 gear, KV would reach 1.5–2.0, requiring a gear twice as large as needed at rated torque. DIN Class 3–5 brings KV down to 1.02–1.05, allowing compact gear design. This is the fundamental reason why high-speed compressor helical gears must be ground to DIN Class 3–5 — not noise, but gear body size.
- Oil mist lubrication system: At v > 80–100 m/s, spray lubrication from a nozzle is unable to deliver oil to the tooth mesh zone because the surface velocity sweeps the oil away before it penetrates. Pressurised injection lubrication with nozzles directing oil into the mesh zone from both sides is required at these velocities.
Helix Angle for High-Speed Compressor Helical Gears
Centrifugal compressor speed increaser helical gears use low helix angles (β = 5–15°) — counter to the general trend favouring higher helix angles for noise reduction. The reason: at very high pitch-line velocities, the axial thrust F_a = F_t × tan β generates significant axial displacement of the rotor shaft that must be absorbed by the shaft thrust bearing. At v = 100 m/s with β = 25°, F_a = 0.466 × F_t — and the thrust bearing at these speeds is a precision tilting-pad bearing whose axial load capacity is limited by its design. The standard API 613 compressor helical gear uses β = 8–15° as a compromise between noise (slightly higher) and manageable axial thrust at high speed.
API 613 Requirements for Compressor Speed Increaser Gears
| API 613 Parameter | Requirement | Rationale |
|---|---|---|
| Gear accuracy class | AGMA 13–15 (≈ DIN Class 3–5) | Controls KV at high speed; limits vibration excitation from TE |
| Vibration limit (bearing housing) | ≤ 25 µm peak-to-peak (or per API 613 Table 3) | Protects impeller tip clearances; prevents rotor-stator contact |
| Gear material | AISI 4340 or equivalent (17CrNiMo6) through-hardened or carburized | High fatigue strength; no risk of fragmentation failure at high speed |
| Lube oil system | Pressurised injection; ISO VG 32–68 turbine oil; duplex filter; oil cooler | Continuous oil flow at all operating conditions; clean oil critical at these tooth velocities |
| Critical speed margin | ≥ 20% separation between running speed and any critical speed | Prevents resonance amplification of TE-induced vibration |
Screw Compressor Timing Gears — Backlash and Phase Accuracy
An oil-free screw compressor requires exact synchronisation of the male and female rotor phases — if the rotors are allowed to contact each other (which happens if the timing gear backlash is too large and the rotors thermally expand to reduce their clearance), the result is immediate rotor seizure and compressor destruction. The timing helical gear specification for oil-free screw compressors therefore has backlash as the primary parameter — not load capacity (the gears transmit light torque only) but DIN 3967 class g or h to maintain rotor clearance throughout the thermal operating range.
The timing gears must also maintain constant phase relationship during thermal cycling of the compressor casing. This requires that the timing gear housing expansion (which changes the centre distance and therefore the backlash) is compensated in the design — either by allowing backlash to increase with temperature within a controlled range that still maintains rotor clearance, or by using temperature-compensated centre distance adjusters on the timing gear housing.
Korea Ever-Power — Compressor Speed Increaser Helical Gear Supply
Korea Ever-Power produces replacement and new-design helical cut gears for centrifugal compressor speed increasers at DIN Class 3–5, Ra ≤ 0.2 µm, in API 613-compatible materials (17CrNiMo6 through-hardened or carburized), with 100% gear analyser measurement and vibration qualification documentation. As a direct helical gear manufacturer, Korea Ever-Power performs the critical speed calculation and vibration estimate for replacement gears, confirming compliance with API 613 vibration limits at the specified running speed before production. For screw compressor timing gears, Korea Ever-Power produces matched pairs with confirmed backlash class and pitch accuracy, verified on the gear analyser. Browse the helical gear product range for compressor applications.
Frequently Asked Questions
Standard gear oil (ISO VG 220–320 with EP additives) is formulated for moderate-speed gearboxes where the EHL film needs the viscosity support of a high-viscosity base. At the pitch-line velocities of compressor speed increasers (50–200 m/s), the oil film is generated by hydrodynamic action at high speed — the EHL film thickness h_min scales approximately with v^0.68. Using ISO VG 220 at 100 m/s generates far more film than needed, causing excessive viscous losses and heat generation. ISO VG 32–68 turbine oil (no EP additives — EP additives are corrosive to the copper alloys in compressor bearings) maintains adequate EHL film while minimising churning losses. The absence of EP additives is acceptable because at these velocities the gear pair is firmly in the full EHL regime — asperity contact does not occur and EP protection is unnecessary.
API 613 adds several requirements beyond ISO 6336: (1) a mandatory vibration acceptance test at full speed and load before shipment, with vibration at bearing housing ≤ 25 µm peak-to-peak; (2) specific rotor dynamics analysis including torsional critical speed, lateral critical speed, and separation margin from all operating speeds; (3) material requirements specifying maximum impurity levels in the gear steel beyond the standard EN/ASTM composition ranges; (4) a witnessed mechanical run test by the purchaser or purchaser’s inspector; (5) specific lube oil cleanliness requirements (ISO 16/14/11 minimum). These additional requirements are why an API 613-compliant compressor helical gear typically costs 2–4× more than a standard industrial gear of the same size.
Yes — provided the noise is from progressive surface fatigue (pitting or micropitting) that has roughened the tooth flanks above the original Ra ≤ 0.2 µm, and provided the remaining case depth after material removal is still within the minimum specified. Korea Ever-Power accepts high-speed compressor helical gears for re-grinding, measures the remaining material condition and case depth, and provides a re-grind feasibility report before committing to the operation. Re-grinding to DIN Class 4–5 from an in-service roughened surface is feasible if the total material removal needed is below 0.2–0.3 mm — beyond this, a replacement gear is more cost-effective than attempting re-grind.
In an integrally geared compressor, the impellers are mounted directly on the shaft extensions of the high-speed pinions inside the gearbox housing — there are no separate compressor casings flanged to a speed increaser. Each pinion can drive two impellers (one on each shaft end) at different pressure ratios, allowing multi-stage compression in one machine. The advantage: no coupling between gearbox and compressor, lower rotating mass, more compact installation. The disadvantage: the gear design is inseparable from the compressor design — the gear is part of the process boundary and must withstand the process gas pressure loads in addition to the gear loads. Korea Ever-Power produces replacement pinion gears for integrally geared compressors with the specific material certifications and dimensional controls required for their dual-function role.
Compressor Speed Increaser or Timing Gear Enquiry
Provide your pinion speed (RPM), pitch diameter, module, helix angle, and API 613 requirement status. Korea Ever-Power produces DIN Class 3–5 compressor gears with Ra ≤ 0.2 µm, full gear analyser documentation, and API 613 vibration estimate — at standard helical gear lead times.
DIN Class 3–5 · Ra ≤ 0.2 µm · API 613 compliant · Vibration estimate · Turbine oil compatible · MOQ 1 piece
Editor: Cxm
