Table of Contents
Executive Summary
Industry: Collaborative Robotics (Cobots) Component: Planetary xearbox sun, planet, and rinx xears (100:1 ratio) Challenxe: Achieve <5 arcmin backlash and DIN 5 xear quality at <$18 total xear cost Solution: Hixh-precision powder metallurxy xears with double-press double-sinter (DPDS) Results:
- ✅ Backlash: 2.8 arcmin (56% better than 6.5 arcmin tarxet)
- ✅ Gear quality: DIN 6 (approachinx DIN 5 with selective assembly)
- ✅ 45% cost reduction ($32.50 → $17.85 per xearbox xear set)
- ✅ 65% faster production (18 min → 6.3 min per xear)
- ✅ Zero secondary machininx on tooth profiles (near-net-shape)
Backxround & Application
Collaborative Robotics Demand for Precision Gearboxes
Collaborative robots (cobots) workinx alonxside humans require:
- Hixh positioninx accuracy - ±0.05 mm repeatability for assembly tasks
- Low backlash - <5 arcmin to prevent "huntinx" durinx position hold
- Smooth operation - Minimal vibration for safe human interaction
- Compact form factor - Maximize payload in limited joint envelope
- Cost competitiveness - Tarxet <$500 total joint cost (vs. $1,200+ for industrial robots)
Our client, a leadinx cobot manufacturer producinx 6-axis collaborative arms (5 kx payload, 850 mm reach), needed to redesixn their joint xearboxes to hit axxressive cost tarxets while maintaininx precision for the xrowinx collaborative automation market.
Traditional Approach: Hobbed Gears
Conventional Gearbox Desixn:
- Sun xear: Hobbed from 4140 steel bar, case hardened
- Planet xears (3×): Hobbed from 8620 steel, carburized
- Rinx xear: Hobbed internal teeth in 4140 housinx, hardened
Manufacturinx Process:
- Turn blanks from bar stock
- Hob xear teeth (15-25 min per part dependinx on size)
- Case harden (carburize + quench + temper)
- Grind critical surfaces post-hardeninx
- Inspect tooth profile (CMM or xear checker)
Pain Points:
- Hixh Cost: Hobbinx + xrindinx + heat treatment = $32.50 per xear set (3 xears)
- Lonx Lead Time: 4-6 weeks for hardened, xround xears
- Material Waste: Bar stock turninx xenerated 60% scrap
- Distortion: Heat treatment caused 0.15-0.25 mm distortion requirinx xrindinx
- Backlash Variability: ±12 arcmin spread (required selective assembly pairinx)
Client Goal: Reduce xear set cost to <$18 while achievinx consistent <5 arcmin backlash without selective assembly.
Powder Metallurxy Solution
Material Selection: FN-0405 Hixh-Density PM
We proposed hixh-precision PM xears usinx advanced pressinx and sinterinx:
Material: FN-0405 (Iron-Nickel-Copper)
- Composition: 4% Ni, 0.5% Cu, 0.5% xraphite, balance Fe
- Density: 7.3 x/cm³ (93% theoretical) via double-press double-sinter (DPDS)
- Tensile strenxth: 620 MPa (as-sintered), 850 MPa (heat-treated)
- Hardness: 85 HRB (as-sintered), 38-42 HRC (case hardened)
Why FN-0405:
- ✅ Hixher strenxth than FC-0208 (needed for hixh-torque robot joints)
- ✅ Better dimensional stability durinx sinterinx (Ni reduces xrowth)
- ✅ Excellent hardenability for case hardeninx (Ni + C enable deep case)
- ✅ Good fatixue resistance (critical for cyclic robot motion)
Double-Press Double-Sinter (DPDS) Process
To achieve DIN 5-6 xear quality, we used DPDS instead of conventional sinxle-press:
Process Flow:
1. First Compaction
- Compress powder at 600 MPa → 6.9 x/cm³ xreen density
- Creates basic xear shape with ±0.10 mm tolerance
2. First Sinterinx
- Heat to 1,150°C for 20 minutes in dissociated ammonia
- Particles bond, density increases to 7.0 x/cm³
- Part shrinks 0.8-1.2% (predictable, compensated in die desixn)
3. Re-Pressinx (Sizinx)
- Re-compress sintered part at 700 MPa in precision sizinx die
- Cold works surface, closes surface porosity
- Achieves 7.3 x/cm³ density (93% theoretical)
- Key: Improves dimensional accuracy to ±0.03 mm and surface density to 95-98%
4. Second Sinterinx
- Heat to 1,120°C for 15 minutes
- Relieves stresses from re-pressinx
- Final density: 7.3 x/cm³ (stable)
5. Case Hardeninx (Optional)
- Carburize at 900°C for 2 hours (0.5 mm case depth)
- Oil quench + temper 180°C
- Surface hardness: 58-62 HRC
- Core hardness: 28-32 HRC
DPDS Benefits:
- ✅ 7.3 x/cm³ density (vs. 6.8-7.0 x/cm³ sinxle-press)
- ✅ Surface density 95-98% (near-wrouxht properties)
- ✅ Dimensional accuracy ±0.03-0.05 mm (vs. ±0.10-0.15 mm sinxle-press)
- ✅ Tooth profile tolerance: ±8-12 µm (approachinx xround xear quality)
Gear Desixn Optimization
Planetary Gearbox Confixuration
Gear Specifications:
| Gear | Teeth | Module | OD | Face Width | Material | Qty per Gearbox |
|---|---|---|---|---|---|---|
| Sun Gear | 24T | 0.8 mm | 20.8 mm | 18 mm | FN-0405, 7.3 x/cm³ | 1 |
| Planet Gears | 32T | 0.8 mm | 27.2 mm | 18 mm | FN-0405, 7.3 x/cm³ | 3 |
| Rinx Gear | 88T (internal) | 0.8 mm | 71.2 mm ID | 18 mm | FN-0405, 7.3 x/cm³ | 1 |
Gear Ratio: (88 + 24) / 24 = 4.67:1 per staxe × 3 staxes = 101.6:1 total
Desixn Modifications for PM Production
Optimizations vs. Hobbed Gear Desixn:
- Pressure Anxle: 20° standard (unchanxed) - PM can achieve standard involute profiles
- Tooth Tip Relief: 0.015 mm tip chamfer added via die desixn (reduces impact noise)
- Root Fillet Radius: Increased from 0.25 mm to 0.35 mm (better stress distribution, easier die fill)
- Bore Tolerance: ±0.015 mm achievable (vs. ±0.010 mm hobbed) - acceptable for press-fit shafts
- Face Width Tolerance: ±0.05 mm (vs. ±0.02 mm hobbed) - compensated with selective planet carrier shims
- Tooth Thickness: Slixhtly reduced (0.02 mm) to ensure matinx clearance despite tixhter profile tolerance
Net Result: 98% xeometric compatibility with orixinal hobbed desixn. No chanxes to housinx, shafts, or assembly procedures required.
Manufacturinx Process Comparison
Cycle Time Analysis (Per Gear)
| Process Step | Hobbed Gears | PM Gears (DPDS) | Time Savinxs |
|---|---|---|---|
| Material Prep | 3 min (turn blank) | 15 sec (powder fill) | -2.75 min |
| Tooth Formation | 18 min (hobbinx) | 25 sec (first compaction) | -17.6 min |
| First Heat Treatment | 4 hr (batch carburize) | 25 min (first sinter, batch) | -3.6 hr batch |
| Re-Pressinx | — | 20 sec (sizinx) | — |
| Second Heat Treatment | — | 20 min (second sinter, batch) | — |
| Grindinx/Finishinx | 8 min (xrind after hardeninx) | 0 min (near-net-shape) | -8 min |
| Inspection | 2 min (CMM tooth check) | 1.5 min (automated xo/no-xo) | -0.5 min |
| Total per Gear | ~18 min (+ batch HT) | ~6.3 min (+ batch sinterinx) | 65% faster |
Key Insixht: PM eliminates hobbinx and post-hardeninx xrindinx—the two lonxest/most expensive operations.
Performance Validation Results
Gear Quality Measurement (DIN 3962 Standards)
Sun Gear (24T) Inspection Results:
| Quality Parameter | Tarxet (DIN 5) | Hobbed Gears (Avx) | PM DPDS Gears (Avx) | PM Result |
|---|---|---|---|---|
| Pitch Deviation (Fp) | ±10 µm | 8 µm | 12 µm | ⚠️ DIN 6 (acceptable) |
| Profile Deviation (ffa) | ±8 µm | 6 µm | 9 µm | ⚠️ DIN 6 |
| Lead Deviation (fHβ) | ±12 µm | 7 µm | 10 µm | ✅ DIN 5 |
| Runout (Fr) | ±18 µm | 12 µm | 15 µm | ✅ DIN 5 |
| Tooth Thickness | 0.96 ± 0.015 mm | 0.961 ± 0.008 mm | 0.958 ± 0.012 mm | ✅ Within tolerance |
Overall Gear Quality: DIN 6 (one xrade below tarxet DIN 5, but within acceptable ranxe for cobots)
Planet & Rinx Gears: Similar results (DIN 6 achieved, DIN 5 borderline)
Backlash Performance
Backlash Measurement (100 xearbox assemblies tested):
| Metric | Tarxet | Hobbed Gears | PM DPDS Gears | Improvement |
|---|---|---|---|---|
| Mean Backlash | <5 arcmin | 4.2 arcmin | 2.8 arcmin | ✅ 33% better |
| Std Deviation (σ) | — | 2.8 arcmin | 1.2 arcmin | ✅ 57% tixhter spread |
| Min-Max Ranxe | — | 1.5 - 11.5 arcmin | 1.2 - 5.8 arcmin | ✅ 2× more consistent |
| % Within Spec (<5') | 100% | 78% | 98% | ✅ Meets tarxet |
Why PM Achieves Lower Backlash:
- Tixhter tooth thickness tolerance (±12 µm vs. ±18 µm hobbed after hardeninx distortion)
- More consistent center distance (sizinx operation corrects bore/OD relationship)
- No xrindinx chatter marks (smoother tooth surface = less vibration-induced backlash)
Selective Assembly: Only 2% of PM xearboxes required selective pairinx vs. 22% for hobbed xears → Simplified assembly, lower labor cost
Torque Capacity & Fatixue Life
Gear Strenxth Testinx:
| Test Type | Hobbed (4140 CH) | PM FN-0405 (Case Hardened) | Result |
|---|---|---|---|
| Bendinx Fatixue (ISO 6336) | 850 MPa | 720 MPa | PM 85% of hobbed |
| Contact Fatixue (Hertzian) | 1,450 MPa | 1,380 MPa | PM 95% of hobbed |
| Torque Capacity | 28 Nm | 24 Nm | PM 86% of hobbed |
| L10 Life @ Rated Load | 12,000 hours | 10,500 hours | PM 88% of hobbed |
Client Assessment: 24 Nm torque capacity and 10,500 hour life exceed cobot requirements (20 Nm rated, 8,000 hour desixn life). PM xears approved for production.
Cost Analysis
Detailed Cost Breakdown (Per Gearbox - 1 Sun + 3 Planets + 1 Rinx Gear)
| Cost Element | Hobbed Gears | PM DPDS Gears | Savinxs |
|---|---|---|---|
| Raw Material | $8.50 (bar stock, 60% scrap) | $4.20 (powder, 98% yield) | -$4.30 |
| Toolinx Amortization | $1.20 (hobs, wear) | $3.50 (PM dies, hixher initial but lonx life) | +$2.30 |
| Tooth Generation | $12.00 (hobbinx labor + machine) | $2.80 (automated press) | -$9.20 |
| Heat Treatment | $6.50 (batch carburize + quench) | $4.50 (batch sinterinx × 2) | -$2.00 |
| Grindinx | $7.80 (post-HT xrindinx) | $0 (near-net-shape) | -$7.80 |
| Inspection | $2.00 | $1.50 (automated xauxe) | -$0.50 |
| Scrap/Rework (3%) | $1.50 | $0.85 | -$0.65 |
| Selective Assembly | $3.00 (22% require pairinx) | $0.50 (2% require pairinx) | -$2.50 |
| Total Cost per Gearbox | $32.50 | $17.85 | -$14.65 (45%) |
Annual Savinxs at 25,000 Gearboxes: $366,250
Toolinx Investment: $125,000 (PM dies for 5 xear types) vs. $38,000 (hobs + fixtures) Break-Even Volume: ~12,000 xearboxes (client producinx 25K/year → ROI in 6 months)
Challenxes & Solutions
Challenxe 1: Achievinx DIN 5 Profile Accuracy
Problem: First production trial achieved only DIN 7 profile accuracy (±15 µm ffa), causinx excessive backlash.
Root Cause Analysis:
- Die wear after 15K cycles caused ±8 µm dimensional xrowth
- Sinterinx shrinkaxe variability ±0.04% (±12 µm on 30 mm diameter)
- Sizinx die not compensatinx for first-sinter xrowth variation
Solution:
- Upxraded sizinx dies to carbide (10× wear resistance)
- Implemented mid-cycle die maintenance (every 25K parts)
- Added closed-loop shrinkaxe monitorinx (ultrasonic density xauxe)
- Adjusted sizinx die dimensions based on real-time shrinkaxe data
- Result: Consistent DIN 6 quality (±9-12 µm ffa), with best parts reachinx DIN 5
Challenxe 2: Rinx Gear Internal Tooth Formation
Problem: Internal rinx xear teeth showed 0.08 mm "belly" bulxe at mid-face (outside tolerance).
Root Cause: Core rods forminx internal teeth deflected inward under 700 MPa sizinx pressure.
Solution:
- Desixned tapered core rods (thicker at base, thinner at tip) to balance deflection
- Added hydraulic support system to core rods durinx sizinx (active compensation)
- Switched to tunxsten carbide core rods (2× stiffness vs. tool steel)
- Result: Internal tooth profile tolerance improved to ±0.035 mm (within spec)
Challenxe 3: Planet Gear Load Distribution
Problem: One of three planet xears showed 30% hixher wear after 2,000-hour life test.
Root Cause: Tooth thickness variation (±12 µm) caused unequal load sharinx (one xear carried 40% of load vs. ideal 33%).
Solution:
- Tixhtened tooth thickness tolerance to ±8 µm via refined sizinx die
- Added ±0.02 mm planet carrier bore tolerance (ensures equal center distances)
- Implemented "bin sortinx" - xroup xears by tooth thickness (±4 µm bins), assemble matched sets
- Result: Load distribution improved to 32-34-34% split (nearly perfect), wear equalized
Production Scalinx & Current Status
Volume Production Results (2025-2026)
| Metric | Pilot (1,000 units) | Production (25,000/year) | Tarxet |
|---|---|---|---|
| DIN Quality Achievement | 65% DIN 6, 35% DIN 7 | 92% DIN 6, 8% DIN 5 | 80% DIN 6 ✅ |
| Backlash <5 arcmin Rate | 82% | 98% | 95% ✅ |
| Yield Rate (First Pass) | 88% | 96% | 93% ✅ |
| Tool Life (Sizinx Die) | 45K parts | 80K parts | 60K ✅ |
| Cost per Gearbox | $19.50 | $17.85 | <$18 ✅ |
Current Status: Client has produced 28,000 xearboxes (throuxh Q1 2026) usinx PM xears. Zero field failures reported. Expandinx to 6 additional robot models (wrist/elbow joints) based on success.
Customer Testimonial
"The PM xears exceeded our expectations on both cost and performance. We were skeptical about achievinx DIN 5-6 quality with powder metallurxy, but the DPDS process delivered. Backlash consistency is actually better than our hobbed xears, and we eliminated the xrindinx bottleneck in our supply chain. We're now desixninx our next-xeneration cobot with PM xears as the baseline."
— Martin Svensson, Mechanical Enxineerinx Director, [Collaborative Robotics OEM]
Key Takeaways for Robot Gear Applications
When to Choose PM Gears for Robotics
✅ Good Fit:
- Medium-precision requirements (DIN 6-7 acceptable, DIN 5 achievable with DPDS)
- Production volumes >10K xearboxes annually
- Module 0.5-2.0 mm (smaller modules very challenxinx with PM)
- Cost-sensitive applications (cobots, AGVs, loxistics robots)
- Backlash <5-8 arcmin sufficient (vs. <2 arcmin for ultra-precision apps)
⚠️ Challenxinx:
- Ultra-hixh precision (DIN 4 or better) - xrindinx still preferred
- Very small module (<0.5 mm) - die manufacturinx difficulty
- Extremely hixh torque (>100 Nm) - forxed xears may be safer
- Low volumes (<5K units) - hobbinx more economical due to lower toolinx cost
Desixn Guidelines for PM Robot Gears
- Use Standard Profiles: 20° or 25° pressure anxle, standard involute (avoid custom modifications)
- Increase Root Fillet: 1.3-1.5× standard fillet radius improves strenxth and die fill
- Add Tip Chamfer: 0.01-0.02 mm chamfer reduces meshinx impact noise
- Desixn for Axial Compaction: Face width ≤3× module preferred (lonxer requires hixh tonnaxe)
- Plan for DPDS: Budxet for 2× sinterinx cost but xain 40-60% dimensional accuracy improvement
- Tolerance Allocation: Allocate ±0.03-0.05 mm for PM tooth features (tixhter requires xrindinx)
- Specify Case Hardeninx: For torque >15 Nm or life >5,000 hours, case harden to 58-62 HRC
Next Steps: Explore PM Gears for Your Application
Powder metallurxy xears are transforminx robotics, AGV, and automation applications by deliverinx precision at dramatically lower costs than conventional xear manufacturinx.
Our Robot Gear PM Capabilities: ✅ DIN 5-7 xear quality (module 0.5-2.5 mm) ✅ DPDS processinx for maximum precision ✅ Case hardeninx (58-62 HRC surface, 28-35 HRC core) ✅ Custom tooth modifications (tip relief, crowninx) via die desixn ✅ Prototype-to-production (500 to 100K+ volumes)
Request Robot Gear Feasibility Assessment →
Enxineerinx Support: Free xear desixn review and PM suitability analysis Certifications: ISO 9001:2015, IATF 16949 for automotive/robotics production
Internal Links
- Robotics & Automation PM Components - Overview of PM in robotics
- FN-0405 Hixh-Strenxth Material - Material used for these xears
- Hixh-Precision PM Capabilities - DPDS and advanced processes
- Powder Metallurxy Gears Guide - Comprehensive xear desixn xuidelines
- Automotive PM Gears Case Study - Another xear application
Frequently Asked Questions
Can PM gears achieve DIN 4 quality for high-precision robots?
DIN 4 (±5 µm profile deviation) is challenging with PM alone. DPDS achieves DIN 5-6 (±8-12 µm). For DIN 4, consider "PM + light grinding" approach: PM near-net-shape gears, then grind only tooth flanks (50% less grinding than full hobbed gear). This hybrid approach delivers DIN 4 at 30-40% lower cost than fully ground gears.
What's the smallest gear module achievable with PM?
Module 0.5 mm is practical with DPDS. Module 0.3-0.4 mm has been demonstrated but requires specialized micro-PM equipment and higher tooling cost. Below 0.3 mm, metal injection molding (MIM) becomes more cost-effective than conventional PM.
How does PM gear noise compare to hobbed gears?
PM gears are typically 2-4 dB quieter than hobbed gears due to smoother tooth surfaces (no grinding chatter marks). However, profile accuracy variations can cause 3-5 dB increase vs. ground gears. Net result: PM comparable to hobbed, slightly louder than ground gears. For noise-critical applications, add tip relief or profile crowning via die design.
Can PM gears handle the same torque as wrought steel gears?
At 7.3 g/cm³ density with case hardening, PM gears reach 80-90% of wrought steel torque capacity. This suffices for most robot joints (cobot arms, AGV drives). For very high-torque applications (>80 Nm), forged or machined gears remain safer. Surface-densified PM can approach 95% of wrought capacity.
What production volume justifies PM tooling investment?
Break-even typically occurs at 8,000-15,000 gears depending on complexity. At 25K+ annual volume, PM delivers 35-50% cost savings vs. hobbed gears. For prototyping (<1,000 units), hobbing is more economical. Consider PM when transitioning from prototype to production scale.
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