Spiral Bevel Gearbox - Engineering Documentation
Changing the direction of power transmission by 90 degrees with high torque and quiet operation, this gearbox is designed to examine axial thrust absorption, backlash adjustment, and lubrication/breathing dynamics with ISO/DIN standards.
Spiral Bevel Gearbox Overview
Changing the direction of power transmission by 90 degrees with high torque and quiet operation, this gearbox is designed to examine axial thrust absorption, backlash adjustment, and lubrication/breathing dynamics with ISO/DIN standards.
This structure is used for analyzing agricultural machinery (PTO drives), conveyor belt drives, mixer gearboxes, and industrial right-angle drives.
What You Will Learn
90-Degree High Torque Transmission
Demonstrates how the spiral form engages multiple teeth simultaneously (high contact ratio) to operate quieter and stronger than straight bevel gears.
Axial Thrust Management
As spiral gears rotate, they try to push each other outward. Explains how this massive force is absorbed by tapered roller bearings and slotted nuts.
Thermal Expansion and Breathing
Highlights how the pressure from expanding hot gear oil is released via a breather plug to prevent the oil seals from blowing out.
Component Engineering Breakdown
Materials, manufacturing methods, surface conditions, tolerances, and functional roles of the major components.
Main Housing
Housing Element
Cast Iron EN-GJL-250 (GG25) | EN 1561 | Mfg: Sand Casting + CNC Machining
External: epoxy-based paint (RAL 7016) | Internal: machined, Ra ≤ 3.2 µm | Bearing bores: precision bored, Ra ≤ 1.6 µm
Bearing bore concentricity: ≤ 0.02 mm (ISO 1101) | Flatness (cover seating surface): ≤ 0.05 mm
Functional Description
The lower chassis block housing all shafts, gears, and lubricating oil. It forms the skeleton of the gearbox and carries all assembly loads.
Critical Points
- Vibration damping cast iron material (EN-GJL-250)
- Bearing bores machined in single setup (concentricity ≤ 0.02 mm)
- Cover seating surface (flatness ≤ 0.05 mm)
Cast iron (GG25) provides excellent vibration damping. The lamellar graphite structure absorbs sound and vibration, reducing gear noise. Bearing bores are machined in a single setup on a CNC machining center to ensure concentricity.
Oil Drain Plug
Maintenance Element
C45 (1.0503) Steel + AlNiCo Permanent Magnet Tip | Mfg: Turning + Magnet Press Fit
Zinc-Nickel plating (Zn-Ni, 240+ hrs salt spray, ISO 9227) | Magnet tip: exposed
Thread: M18x1.5 – 6g (ISO 965-1) | Sealing: tapered thread (NPTF) or Dowty seal
Functional Description
Located at the lowest point of the housing, it allows old oil to be drained during maintenance. Its magnetic tip collects wear particles.
Critical Points
- Magnetic tip capturing wear particles (AlNiCo)
- Zn-Ni corrosion protection (240+ hrs salt spray)
These plugs are typically manufactured with a Magnetic tip. Micro steel particles that break off as the gears run are collected on the magnet, preventing them from circulating in the oil and destroying the bearings. Must be cleaned during every oil change.
Plug Washer (Bonded Seal / Dowty Seal)
Static Sealing
Steel outer ring + NBR elastomer inner ring (Dowty type) | Alternative: Annealed Copper (Cu-ETP, EN 1652)
Zinc plated steel ring, black NBR
Crush capacity: NBR ring 25-30% compression ratio | Single-use (work-hardening for annealed copper)
Functional Description
A sealing washer whose elastomer ring crushes when the drain plug is tightened, preventing oil leaks from the threads.
Critical Points
- Crush-forming elastomer sealing ring
- Single-use; must be renewed at every oil change
Dowty (bonded seal) washers provide sealing at lower torque compared to copper washers and are more resistant to vibration. Copper washers work-harden after being crushed once and must not be reused.
Spiral Bevel Pinion Gear
Gear Element
18CrNiMo7-6 (1.6587) Case-Hardening Steel | EN 10084 | Mfg: Hobbing + Gleason/Klingelnberg Profile Grinding
Carburizing (Case Hardening) 58-62 HRC, effective case depth (Eht) 0.8-1.2 mm | Tooth flanks ground, Ra ≤ 0.4 µm
Gear quality: ISO 17485 Class 6 (spiral bevel) | Contact pattern: 70-80% surface contact | Pinion-crown matched set: lapped, not individually replaceable
Functional Description
The small conical gear located at the end of the input shaft, transmitting torque to the crown gear at a 90-degree angle. The spiral structure softens sudden shock loads.
Critical Points
- Spiral involute tooth profile (ISO 17485 Class 6)
- Case-hardened surface (58-62 HRC, Eht 0.8-1.2 mm)
- Pinion-crown matched set (individually non-replaceable)
Spiral bevel gears have a higher contact ratio than straight bevel gears, providing quieter operation and higher torque capacity. Pinion and crown gear are manufactured and lapped as a matched set at the factory. If one gets damaged, both must be replaced. During assembly, shims are used to precisely adjust backlash between 0.08-0.15 mm.
Input Shaft
Power Transmission Element
42CrMo4 (1.7225) Heat-Treatable Steel | EN 10083-3 | Mfg: Forging + CNC Turning + Centerless Grinding
Bearing surfaces: ground, Ra ≤ 0.2 µm, hardness 50-55 HRC (induction) | Keyway: milled, JS9 tolerance
Bearing fit diameters: k5 or m5 (ISO 286-2) | Runout: ≤ 0.01 mm | Cylindricity: ≤ 0.005 mm | Dynamic balance: ISO 21940-11 G6.3
Functional Description
The main shaft that transmits rotational motion from the motor or drive source into the gearbox. It rotates at the highest RPM in the system.
Critical Points
- Keyway slot (JS9, DIN 6885-1)
- Induction hardened bearing surface (50-55 HRC)
- Dynamic balance (ISO 21940 G6.3)
The input shaft typically rotates at the highest RPM in the system; therefore, balancing and fatigue strength are of critical importance. 42CrMo4 heat-treatable steel offers high torsional strength and good machinability. Bearing seats are induction hardened for increased wear resistance.
Input Shaft Bearing (Tapered Roller)
Bearing Element
100Cr6 (1.3505) Bearing Steel | ISO 683-17 | Cage: pressed steel or polyamide
Rings and rollers: superfinished, Ra ≤ 0.1 µm | Heat treatment: 58-62 HRC
Radial internal clearance: C2 or Normal (ISO 5753) | Tapered Roller type: single row, back-to-back mounting (X or O arrangement)
Functional Description
The tapered roller bearing that supports the input shaft and counteracts the axial thrust forces generated by the spiral gears.
Critical Points
- Axial thrust load absorption
- Tapered roller type – combined load capacity
- X or O arrangement mounting
Spiral bevel gears create massive axial thrust forces. Therefore, Tapered Roller Bearings, which can handle combined loads (radial + axial) in both directions, are used instead of standard ball bearings. Two bearings are mounted back-to-back (X or O arrangement) to absorb thrust in both directions.
Retaining Ring
Axial Fastening Element
C67S (1.1231) Spring Steel | DIN EN 10132-4 | Mfg: Wire Forming + Heat Treatment
Phosphating + anti-rust oil (DIN 471 compliant)
Shaft groove tolerance: JS9 (DIN 471) | Ring thickness tolerance: ±0.02 mm
Functional Description
A retaining ring that limits the axial movement of parts on the shaft and fixes the position of the pinion gear.
Critical Points
- Axial lock (DIN 471)
- Phosphated spring steel – corrosion resistant
The ring counteracts the axial forces acting on the pinion gear. To withstand centrifugal forces at high RPMs, the groove depth and ring thickness must conform to DIN 471 standard.
Shaft Thrust Washer
Load Distributor
C45 (1.0503) Hardened and Ground Steel | Hardness: 45-50 HRC
Ground, Ra ≤ 0.8 µm | Parallel faces
Thickness tolerance: ±0.02 mm | Parallelism: ≤ 0.01 mm
Functional Description
A hardened spacer ring that distributes surface pressure between the slotted nut and the bearing inner race.
Critical Points
- Surface pressure distribution
- Hardened and ground (45-50 HRC)
While adjusting the bearing preload with the slotted nut, this washer prevents the rotational motion of the nut from damaging the bearing race. Its hardened surface resists deformation under high contact pressure.
Slotted / Castle Nut
Axial Fastening and Preload Element
C45 (1.0503) or 42CrMo4 (1.7225) Steel | Mfg: Turning + Milling (slots)
Phosphate coating + lubrication
Thread: M20x1.5 – 6H (ISO 965-1) | Preload torque: per manufacturer specification (typical 40-60 Nm)
Functional Description
The slotted nut that adjusts the bearing clearance (preload) on the input shaft and secures the shaft. A split pin is passed through its slots to prevent loosening.
Critical Points
- Bearing Preload adjustment
- Split pin slots – mechanical anti-loosening lock
How tightly the bearings rotate is determined by tightening this nut (Preload adjustment). Over-tightening shortens bearing life; under-tightening causes backlash. After adjustment, a split pin is passed through the nut slots to create a mechanical lock.
Split Pin / Cotter Pin
Safety Element
Mild Steel (EN 10204 2.2) or Brass (CuZn37, EN 12164) | Mfg: Wire Forming
Zinc plating (steel) or natural (brass)
Diameter: matched to shaft hole (ISO 1234) | Single-use; must be replaced after removal
Functional Description
A locking wire that physically prevents the slotted nut from loosening due to vibration. Passed through the hole in the shaft and the slot of the nut, its ends are bent to form a mechanical lock.
Critical Points
- Mechanical anti-loosening lock (ISO 1234)
- Single-use – must be renewed after removal
Split pins conforming to ISO 1234 are designed to prevent nuts from loosening in vibrating environments. They must be replaced after removal because they deform upon installation.
Input Shaft Oil Seal
Rotary Sealing Element
FKM (Viton, FPM) Elastomer + Garter Spring (stainless steel) | Alternative: NBR (Nitrile), lower temperature limit
Elastomer lip: smooth molded, Ra ≤ 0.1 µm | Outer diameter: coated (corrosion resistance and ease of assembly)
Shaft surface roughness: Ra 0.2-0.4 µm (compatible with seal) | Shaft hardness: ≥ 45 HRC | Seal seating tolerance: H8 (ISO 286-2)
Functional Description
A spring-loaded lip seal preventing gearbox oil from leaking out and dust/moisture from entering at the high-speed input shaft point.
Critical Points
- FKM lip resistant to high speed and temperature
- Stainless steel Garter Spring – constant contact pressure
High RPMs on the pinion shaft generate significant heat at the seal lip. If the system runs continuously at high speed (> 3000 rpm) or high oil temperature (> 100 °C), FKM (Viton) material must be selected for resistance to burning and chemical degradation. NBR seals operate between –30°C to +100°C; FKM seals withstand –20°C to +200°C.
Protective Seal Ring (V-Ring / Dust Deflector)
Outer Sealing and Protection
NBR or FKM Rubber | Mfg: Injection Molding
Flexible lip profile, tight fit on shaft
Selected based on shaft diameter (interference fit, typical 5-8% undersize)
Functional Description
A rotating slinger mounted in front of the main oil seal that rotates with the shaft and flings coarse dust, mud, and foreign matter outward by centrifugal force.
Critical Points
- Centrifugal dust and mud deflector
- Labyrinth seal protecting the main oil seal
Especially in agricultural and construction equipment, it prevents mud from destroying the main seal. The V-ring rotates with the shaft and its lip contacts a stationary surface (e.g., seal flange) to form a labyrinth seal.
Spiral Bevel Crown Gear
Gear Element
18CrNiMo7-6 (1.6587) Case-Hardening Steel | EN 10084 | Mfg: Forging + Hobbing + Gleason Profile Grinding
Carburizing 58-62 HRC, effective case depth 0.8-1.2 mm | Tooth flanks ground, Ra ≤ 0.4 µm
Gear quality: ISO 17485 Class 6 | Lapped matched set with pinion | Backlash: 0.08-0.15 mm (adjusted via shims)
Functional Description
The large wheel that turns the motion from the pinion by 90 degrees and transmits it to the output shaft with high torque. Its spiral structure provides high contact ratio and quiet operation.
Critical Points
- Matched Set production with pinion
- Case-hardened tooth surface (58-62 HRC)
- Backlash 0.08-0.15 mm adjustment range via shims
Crown and pinion gears are always manufactured and lapped as a matched set at the factory. If one gets damaged, both must be replaced together. The crown gear is connected to the output shaft via a key or spline and secured axially with a retaining ring or nut.
Double Output Shaft
Power Transmission Element
42CrMo4 (1.7225) Heat-Treatable Steel | EN 10083-3 | Mfg: Forging + CNC Turning + Keyway Milling
Bearing seats: ground, Ra ≤ 0.2 µm, induction hardening 50-55 HRC | Keyway: JS9
Bearing fit diameters: k5/m5 (ISO 286-2) | Runout: ≤ 0.015 mm | Cylindricity: ≤ 0.005 mm | Keyway symmetry: 0.05 mm
Functional Description
The main drive shaft that receives high torque from the crown gear and can output it from both sides of the housing. Used in applications where power needs to be transmitted in both directions.
Critical Points
- Dual-sided power output
- Induction hardened bearing seats (50-55 HRC)
- Keyway JS9 (DIN 6885-1)
Double-output shafts are especially used in agricultural machinery or conveyor belts where power needs to be transmitted in two directions from a single motor. 42CrMo4 steel provides high torsion and bending strength. The keyway is machined to DIN 6885-1 JS9 tolerance.
Output Shaft Bearing – Left (Tapered Roller)
Bearing Element
100Cr6 (1.3505) Bearing Steel | ISO 683-17
Rings and rollers: superfinished, Ra ≤ 0.1 µm, 58-62 HRC
ISO P0 (Normal) precision class (ISO 492) | Radial internal clearance: C3 (thermal expansion allowance)
Functional Description
Supports the left side of the output shaft and centers the crown gear. The tapered roller type also absorbs axial loads.
Critical Points
- Gear deflection prevention bearing
- C3 radial clearance – thermal expansion allowance
Prevents the crown gear from deflecting, keeping the backlash constant. C3 clearance provides thermal expansion allowance to prevent bearing seizure at operating temperature.
Seal Flange 2 (Left Output Seal Flange)
Housing and Sealing
EN AC-44100 (EN 1706) Aluminum Casting | Alternative: GG20 Cast Iron
Machined seal bore: Ra ≤ 1.6 µm | Gasket seating surface: Ra ≤ 3.2 µm
Seal bore diameter: H8 (ISO 286-2) | Housing mounting hole position tolerance: Ø0.1 mm
Functional Description
A flange that closes the left-side output point of the shaft, contains the oil seal, and is bolted to the housing.
Critical Points
- Seal mounting bore (H8 tolerance)
- Bearing outer race retaining surface
The flange applies axial pressure on the bearing outer race to prevent it from rotating (fretting) inside the housing. It also provides a precision bore for mounting the seal.
Output Oil Seal – Left
Rotary Sealing
NBR (Nitrile) Elastomer + Garter Spring | Temperature range: –30°C to +100°C
Elastomer lip: smooth molded | Outer diameter: coated
Seal seating tolerance: H8 | Shaft surface roughness: Ra 0.2-0.4 µm
Functional Description
The sealing element on the left side of the output shaft. Prevents gearbox oil from leaking out.
Critical Points
- Sealing barrier (NBR)
- Spring-loaded lip – constant contact pressure
NBR seal is suitable for standard mineral oils. For synthetic oils or high temperature (> 100 °C), FKM seal should be preferred.
Flange Screws – Left
Fastener
Grade 8.8 Steel | DIN EN ISO 898-1 | Mfg: Cold Forging + Thread Rolling
Zinc-Nickel plating (Zn-Ni, 240+ hrs salt spray, ISO 9227)
Thread: M8x1.25 – 6g/6H (ISO 965-1) | Tightening torque: 25-28 Nm (dry, µ=0.12) | Cross-pattern tightening mandatory
Functional Description
A group of hex head screws that secure the left seal flange to the housing.
Critical Points
- Grade 8.8 (DIN EN ISO 898-1)
- Cross-pattern tightening – even sealing
For even sealing and to prevent flange distortion, the screws must be tightened in a cross-pattern (star pattern) in incremental steps. Grade 8.8 provides sufficient strength for medium-duty applications.
Output Shaft Bearing – Right (Tapered Roller)
Bearing Element
100Cr6 (1.3505) Bearing Steel | ISO 683-17
Rings and rollers: superfinished, Ra ≤ 0.1 µm, 58-62 HRC
ISO P0 precision class | Radial internal clearance: C3
Functional Description
Supports the right side of the output shaft and absorbs radial/axial loads acting on the housing.
Critical Points
- Radial and axial load support
- C3 clearance – thermal expansion
Works as a matched pair with the left bearing. X or O arrangement mounting absorbs axial thrust in both directions.
Seal Flange 1 (Right Output Seal Flange)
Housing and Sealing
EN AC-44100 (EN 1706) Aluminum Casting | Alternative: GG20 Cast Iron
Machined seal bore: Ra ≤ 1.6 µm | Gasket seating surface: Ra ≤ 3.2 µm
Seal bore diameter: H8 | Housing mounting hole position tolerance: Ø0.1 mm
Functional Description
A flange that closes the right-side output point of the shaft and contains the oil seal.
Critical Points
- Bearing retaining surface
- Seal mounting bore H8
This cover also presses against the bearing outer race to keep the bearing axially fixed. This pressure prevents the bearing from rotating inside the housing (fretting corrosion).
Output Oil Seal – Right
Rotary Sealing
NBR (Nitrile) Elastomer + Spring | Temperature: –30°C to +100°C
Elastomer lip: smooth molded
Seal seating tolerance: H8 | Shaft surface: Ra 0.2-0.4 µm
Functional Description
The sealing element on the right side of the output shaft.
Critical Points
- Sealing barrier (NBR)
- Spring-loaded lip contact
NBR seal is sufficient for standard applications. FKM seal is recommended for high speed (> 3000 rpm) or high temperature conditions.
Flange Screws – Right
Fastener
Grade 8.8 Steel | DIN EN ISO 898-1
Zinc-Nickel plating (Zn-Ni)
Thread: M8x1.25 – 6g/6H | Tightening torque: 25-28 Nm | Cross-pattern and staged tightening
Functional Description
A group of screws that secure the right seal flange to the housing.
Critical Points
- Grade 8.8, Zn-Ni plating
- 3-stage cross-pattern tightening procedure
The screws must be tightened in a cross-pattern sequence in 3 stages (first 50%, then 80%, finally full torque) during assembly.
Top Cover
Housing Element
EN-GJL-250 (GG25) Cast Iron | EN 1561 | Alternative: EN AC-44100 Aluminum
Machined sealing face Ra ≤ 1.6 µm | External: painted (RAL 7016)
Flatness: ≤ 0.05 mm (ISO 1101) | Hole position tolerance: Ø0.15 mm
Functional Description
The main cover providing access to the internal assembly and keeping the oil contained. It also houses the mounting point for the breather plug.
Critical Points
- Rigid cast iron structure (EN-GJL-250)
- Sealing face (flatness ≤ 0.05 mm)
- Breather plug mounting point
A liquid gasket (anaerobic or RTV silicone) or precision-cut fiber gasket is typically used between the cover and main body. When using liquid gasket, the surfaces must be oil-free and clean; otherwise the sealant will not adhere and oil leakage will occur.
Cover Mounting Screws
Fastener
Grade 8.8 Steel | DIN EN ISO 898-1 | Mfg: Cold Forging + Thread Rolling
Zinc-Nickel plating (Zn-Ni, 240+ hrs ISO 9227) | Alternative: Geomet 500
Thread: M8x1.25 – 6g/6H (ISO 965-1) | Tightening torque: 25-28 Nm | Cross-pattern (star pattern) sequence
Functional Description
A group of hex head screws that secure the top cover to the main housing. Must be torqued in a star pattern for even sealing.
Critical Points
- Grade 8.8 (DIN EN ISO 898-1)
- 3-stage cross-pattern tightening (25-28 Nm)
- Zn-Ni corrosion protection
Screws must be tightened incrementally (3-stage) in a cross-pattern: first all screws at 50% torque, then 80% torque, finally full torque (25-28 Nm). This method prevents cover distortion and leakage.
Breather Plug / Vent
Pressure Relief and Ventilation Element
Brass (CuZn39Pb3, EN 12164) Body + Sintered Bronze Filter Element (pore size: 40-60 µm)
Natural brass (nickel plating optional) | Filter: sintered bronze, self-lubricating
Thread: M16x1.5 – 6g (ISO 965-1) | Air permeability: 10-20 L/min (at 1 bar pressure difference) | Water tightness: IP66 (dust and pressurized water)
Functional Description
A filtered valve installed at the highest point of the housing that balances internal pressure while preventing dust/water ingress.
Critical Points
- Pressure relief valve preventing Seal Blow-out
- Sintered bronze filter (40-60 µm pores) – dust and waterproof
- IP66 protection class
As the gearbox operates, the oil heats up and the internal air volume expands. If it were a completely sealed box, the increasing pressure would blow out the weakest points: the oil seals. The breather plug vents this pressure to the atmosphere in a controlled manner, while the sintered bronze filter prevents dust and moisture from entering. When the sump oil temperature reaches 80-100 °C, internal pressure can increase by 0.3-0.5 bar.
Training Scope
This interactive module is intended for technical training, engineering visualization, component identification, and mechanical-system analysis. Numerical values, material selections, tolerances, and manufacturing details shown for individual components should be treated as representative technical information unless a specific manufacturer, drawing, or applicable standard is explicitly identified.