Views: 0 Author: Site Editor Publish Time: 2026-07-20 Origin: Site
The small crankshaft – though compact in size – is the most critical transmission component in a reciprocating compressor. Its manufacturing quality directly determines the compressor's efficiency, service life, and reliability. In key applications such as industrial refrigeration, chemical processing, and air power, every rotation of the crankshaft carries the operational mission of the system. A high quality small crankshaft demands near zero deviation across dozens of processes from material selection, forging, heat treatment, to precision machining. This article systematically reviews the complete technical chain from blank to finished product at Shanghai Totem Machinery Co., Ltd., and shares key control points during production.
Small crankshafts for reciprocating compressors operate under severe conditions of high speed reciprocating motion. The service environment imposes extremely stringent requirements on material strength, surface quality, and geometric accuracy:
Alternating loads: the crankpin is subjected to two impacts per revolution, with cyclic stresses exceeding 200 MPa; fatigue failure is the primary risk.
High rotational speed: industrial compressor speeds typically range from 750 to 1500 r/min, with small models reaching 3000 r/min; centrifugal forces combined with alternating loads intensify stress concentrations.
Lubrication challenge: the journal and connecting rod bearing operate with a thin oil film; surface roughness Ra ≤ 0.4 μm, with minimum oil film thickness only 2–5 μm.
Wear sensitivity: journal wear failure directly leads to compressor scrapping; repair costs account for 40%–60% of the total machine value.
Temperature fluctuations: operating ambient temperature ranges from –20℃ to 80℃; the material must offer good dimensional stability and low temperature toughness.
Corrosive environment: compressors used in chemical service often contact sulfur and chlorine containing media; journal surfaces require adequate corrosion resistance.
Material selection must comprehensively consider strength, toughness, hardenability, machinability, and cost. The following table compares common material options:
Parameter | Recommended Specification | Remarks |
Base material | 40CrNi2MoA (GB/T 3077) | First choice for high loads; excellent hardenability and high fatigue strength |
Alternative A | SNCM439 (JIS G4104) | Japanese standard, equivalent performance, suitable for replacement of imported units |
Alternative B | 42CrMo (GB/T 3077) | Economical option, suitable for medium-load conditions |
Alternative C | 18CrNiMo7-6 (EN 10083) | European standard, carburising steel, more uniform case hardening layer |
Quenched and tempered hardness | 28–34 HRC | Ensures core toughness, balances strength and machinability |
Journal surface hardness | 54–58 HRC | Surface hardening by induction or nitriding; case depth 0.5–1.0 mm |
Accuracy grade | IT7 (H7/h7) | Journal tolerance control to ensure fit clearance |
Surface roughness | Ra ≤ 0.4 μm | Working surface of journal; after honing can reach Ra 0.2 μm |
Due to small cross section and thin walls, the material must combine hardenability with low stress sensitivity. The low quench-cracking tendency of 40CrNi2MoA is particularly suitable for thin-walled structures, and its nickel content effectively improves low-temperature impact toughness. For corrosive service, nitriding or the use of stainless steel substrate (e.g., 17-4PH) may be considered, but the cost and processing difficulty need to be weighed. For economical batch production, 42CrMo performs reliably under medium loads and reduces material cost by approximately 30%.
Reciprocating compressor
The manufacturing process of a small crankshaft is a multi-step, multi-parameter precision chain; deviation in any step may scrap the entire part. The following sections elaborate on blank production, heat treatment and machining.
Forging is the first critical step, directly affecting metal flow line distribution and internal structural density. Die forging is recommended, with a forging ratio ≥ 3, ensuring complete flow lines and uniform structure.
Billet cutting: electric furnace heating at 1180℃, holding ≥ 60 min to ensure temperature uniformity.
Forging: starting temperature 1150℃, finishing temperature ≥ 850℃ to prevent overheating and cracking.
Slow cooling after forging to below 600℃ to avoid residual stresses from rapid cooling.
Spheroidising annealing (optional): 760℃ holding for 4h then furnace cooling to improve machinability and reduce hardness to 180-200 HB.
For small batch trials, open-die forging plus CNC rough turning can reduce tooling costs; for high volume production (> 5000 pieces/year), die forging can lower unit cost by over 40%.
Normalising →Rough machining → Quenching and tempering (860–880℃) + High temperature tempering at 560–600℃) → Semi-finishing → Journal surface hardening → Low temperature tempering.
Quenching and tempering: obtains tempered sorbite structure, provides good machinability for subsequent operations, and ensures core toughness.
Journal surface hardening: induction hardening or nitriding, hardness gradient within 0.5 - 1.0 mm from 58 HRC at the surface to 30 HRC at the core.
Distortion control: quenching distortion of the small crankshaft should be ≤ 0.03 mm; dedicated fixturing is required, and a multi-point support quenching jig is recommended.
Subzero treatment (optional): –80℃ holding for 2 h to eliminate retained austenite and improve dimensional stability.
Tempering process optimisation: multiple low-temperature tempering cycles (180℃ × 2 h × 2 times) effectively relieve surface quenching stresses.
The following table lists typical machining steps and key control parameters:
Operation | Content | Key Control Points |
Rough turning | Rough machining of crank web, main journal, and crankpin | Leave allowance 2-3 mm; control clamping distortion |
Semi-finish turning | Finish turn main journal to IT8 | Surface roughness Ra ≤ 1.6 μm |
Finish turning | Eccentric turning of crankpin, maintain crank radius | Eccentricity tolerance ±0.02 mm |
Grinding | Precision grinding of main journal and crankpin | IT7 accuracy, Ra ≤ 0.4 μm |
Boring | Machining of crankpin bore and oil holes | Coaxiality ≤ 0.02 mm |
Honing (optional) | Plateau honing of journal surface | Improves oil film load capacity; plateau angle 45° |
Dynamic balancing | Weight removal or build up welding | G6.3 balance grade (G2.5 for high speed applications) |
Polishing | Mirror finishing of journal surface | Ra ≤ 0.2 μm, improves fatigue strength |
Inspection | Dimensions, hardness, magnetic particle testing | Verify all items per drawing; 100% MPI |
Thin-wall distortion: crank web thickness is small; clamping force must be precisely controlled. Recommend V-shaped soft jaws plus hydraulic centring fixture.
Quenching distortion compensation: reserve compensation allowance; build a database of measured values; correct 0.01–0.02 mm per batch.
Dynamic balancing precision: small mass makes weight removal require micron-level control; recommend internal bore milling rather than drilling.
Intersecting oil holes: burrs easily form at intersections of angled and straight oil holes; require dedicated deburring tools or electropolishing.
Surface integrity: grinding burns cause residual tensile stresses and reduce fatigue life; control grinding parameters and inspect surface condition.
Our quality inspection runs through every critical step of crankshaft manufacturing and is central to ensuring high production yield. A complete inspection system should cover four dimensions: dimensional accuracy, surface quality, mechanical properties, and non-destructive testing.
Dimensional accuracy | Journal diameter | Micrometer / roundness tester | IT7, roundness ≤ 0.008 mm |
Dimensional accuracy | Coaxiality | Coordinate Measuring Machine (CMM) | ≤ 0.02 mm |
Dimensional accuracy | Crank radius | Special gauge | ±0.05 mm |
Dimensional accuracy | Crank angularity | Dividing head + dial indicator | ±0.5° |
Surface quality | Roughness | Profilometer | Ra ≤ 0.4 μm (after honing ≤ 0.2 μm) |
Surface quality | Grinding burn | Acid etching / eddy current testing | No burn marks |
Mechanical properties | Hardness | Rockwell tester | Journal 54–58 HRC, core 28–34 HRC |
Mechanical properties | Case depth | Metallographic / hardness gradient method | 0.5–1.0 mm |
Non Destructive Testing (NDT) | Surface cracks | Magnetic particle inspection (MPI) | No crack indications |
Non Destructive Testing (NDT) | Internal defects | Ultrasonic testing | GB/T 6402 Grade II |
It is advisable to establish a three-level inspection system: first-piece inspection → in-process sampling → final inspection. First-piece inspection covers all items; in-process sampling remeasures critical dimensions at a 10% rate; final inspection performs 100% MPI and full critical dimension checks. All inspection data should be entered into a traceability system for individual part traceability.
Crankshaft failure is rarely due to a single factor, but results from multiple overlapping causes material, process, and service conditions. Understanding typical failure modes helps to avoid risks at the design and manufacturing stages.
Fatigue fracture: stress concentration at the crankpin fillet radius is the most common origin of fatigue cracks. Preventive measures include increasing the fillet radius (R ≥ 0.5 mm), applying rolling strengthening, and ensuring residual compressive stress on the surface.
Journal wear: rupture of the oil film leads to direct metal-to-metal contact, causing rapid wear. Preventive measures include ensuring surface roughness, using plateau honing to improve oil film retention, and selecting appropriate bearing clearance.
Quench cracking: excessively rapid cooling or high hydrogen content in the material can cause cracks. Preventive measures include optimising quenching media, using isothermal or step quenching, and controlling hydrogen content in the raw material.
Dynamic imbalance: non-uniform mass distribution leads to vibration at high speed. Preventive measures include dynamic balancing of each piece after machining, internal bore milling for weight removal, and controlling blank mass deviations.
Oil hole fatigue: stress concentration at the oil hole edge initiates cracks. Preventive measures include chamfering and polishing the oil hole edge, applying shot peening, and optimising oil hole location away from high-stress zones.
Finish Turning Both Ends
Take a small crankshaft with a main journal diameter of Φ32 mm used in a 1.5 m³/min piston compressor for chemical process gas compression, with an annual output of about 3000 pieces and a design life ≥ 20,000 hours.
Material: 40CrNi2MoA (GB/T 3077), die-forged blank, forging ratio 3.5.
Critical dimensions: main journal Φ32H7, crankpin Φ28H7, crankpin bore Φ16H7.
Crank radius: 22 mm; crank angle: 120° (three cylinder); eccentricity tolerance ±0.02 mm.
Production cycle: forging → quenching and tempering (3 days) → machining (5 days) → surface treatment (1 day) → inspection and warehousing (1 day).
Measured results:
Journal roundness: 0.005 mm (requirement ≤ 0.008 mm).
Surface roughness: Ra 0.32 μm (after honing).
Hardness: journal 56 HRC, core 32 HRC, case depth 0.7 mm.
Dynamic balance: G6.3 achieved (measured G2.8).
Magnetic particle inspection: 100% passed, no crack indications.
The first batch achieved a yield of 98.5%, with only 2 pieces returned for re-boring due to misaligned oil holes. After process optimisation (adding a locating fixture for oil holes), subsequent batches achieved a stable yield above 99.5%.
The manufacturing cost of a small crankshaft is influenced by material, process route, and batch size. The following is a typical cost breakdown (for Φ32 mm crankshaft, annual output 3000 pieces):
Cost Item | Proportion | Remarks |
Material | ~25% | 40CrNi2MoA forged blank |
Forging | ~15% | Die tooling amortisation + forging labour |
Heat treatment | ~12% | Quenching & tempering + surface hardening |
Machining | ~35% | Turning, grinding, boring, honing, etc. |
Inspection | ~8% | Dimensional + NDT |
Other | ~5% | Administration, packaging, transport |
Optimisation suggestions: ① For batches > 5000 pieces/year, die forging is superior to open-die forging, reducing unit cost by 30%. ② Adopt near-net-shape forging to reduce machining allowance and lower machining cost by about 20%. ③ Combine operations (e.g., turn-grind integration) to shorten lead time and reduce work-in-process. ④ Build a quenching distortion database to progressively reduce compensation allowance and improve material utilisation.
Overall hardenability optimisation: vacuum heat treatment with precise carbon potential control, surface carbon content 0.35–0.42%, hardness uniformity ±1 HRC.
Laser hardening: an upgrade from induction hardening; case depth controllable to 0.3–1.2 mm with minimal heat-affected zone.
3D printing repair: laser cladding + precision machining for worn journals, saving over 60% cost; repaired performance reaches 95% of new parts.
In-process inspection: integrated laser measurement during machining, real-time feedback on crankshaft bending, closed-loop adjustment of cutting parameters.
Composite material crankshaft: carbon fibre + metal hybrid structure, weight reduction of 40%, now in laboratory validation.
Green manufacturing: use of environmentally friendly quench media (polymer water solution replacing mineral oil), reducing waste liquid treatment cost by 50%.
The small crankshaft for reciprocating compressors may be unassuming, but it is the heart of the machine. From material selection, forging forming, heat treatment strengthening, to precision machining – every step must be perfected to the extreme. A qualified small crankshaft embodies the deep integration of materials science, heat treatment technology, precision manufacturing, and quality management. We continuously optimise process parameters so that every small crankshaft meets the “zero-defect” mass-production standard, safeguarding the long-term stable operation of compressors.
