1. The Field Problem: Pits on the Race After Two Years of "Normal" Operation

A stacker-reclaimer gearbox backstop, 6,500 N·m class, was opened at a scheduled overhaul after 26 months. The inner race showed a band of shallow pitting about 8 mm wide, exactly where the sprags sit when locked, with matching crescents on the sprag contact faces. Measured backlash was still within tolerance, the oil was clean, and the bearing had never slipped. By any functional definition the unit was still working — but the pitted band is the first stage of a failure that ends in spalling, sprag skid and, eventually, a runaway belt.

Contact fatigue in sprag backstops is a slow, silent, predictable process. It is also the failure mode that distinguishes a heavy-duty backstop from a catalogue page with a large torque number.

2. Mechanism: From Hertzian Stress to a Pit

2.1 The contact is a line, and the stress is not small

Each sprag touches the inner and outer race on a crowned line. The peak Hertzian pressure is:

p_max = √( w·E′ / (π·R′) )

with w = load per unit contact length, E′ = equivalent elastic modulus (about 113,000 N/mm² for steel on steel) and R′ = equivalent radius of curvature.

Take a sprag carrying 3.0 kN over a 12 mm contact length (w = 250 N/mm), sprag crown radius 6 mm against a 40 mm race radius, so R′ = 5.2 mm:

p_max = √( 250 × 113,000 / (π × 5.2) ) ≈ 1,310 MPa

For heavy-duty units operating near rated torque, with fewer sprags sharing load, 2,000–3,000 MPa is normal. Through-hardened 100Cr6 or case-hardened 20CrMnTi races are specified at 59–62 HRC partly because the allowable contact stress sits only modestly above these values — the design margin lives in hardness and surface quality, not in size.

2.2 Maximum shear sits below the surface

The dangerous stress is not at the surface where you can see it. Maximum alternating shear stress occurs at roughly 0.5–0.8 times the contact half-width below the surface, at about 0.3 p_max. At 1,310 MPa surface pressure that is roughly 400 MPa of alternating shear at a depth of tens of microns — precisely where non-metallic inclusions, carbide networks and grinding burns live.

2.3 Initiation, growth, spall

1. Initiation: a subsurface inclusion or a surface defect (grinding mark, handling dent, debris indentation) concentrates shear.
2. Micro-crack growth: cracks propagate parallel to the surface, then turn up and break out.
3. Pit / spall: a 0.1–0.5 mm crater appears. Contact area shrinks, load per remaining sprag rises, and the process accelerates.
4. Secondary damage: the pit edge skids on the sprag, destroying the oil film and generating debris that indents everything downstream.

2.4 What accelerates it in real plants

• Uneven sprag loading from misalignment — cuts effective contact count and multiplies w.
• Oil film collapse at zero speed. The backstop spends its locked life at zero entrainment velocity; the film is boundary, not hydrodynamic, so surface finish and additive package decide everything.
• Hardness shortfall. A race at 56 HRC instead of 61 HRC can lose half its contact-fatigue life without any visible difference on the certificate.
• Debris indentation. A 20 µm particle pressed into the race creates a stress raiser far more damaging than the particle size suggests.

3. Worked Example: Where the Margin Actually Goes

Read the table as a warning: the torque calculation can be perfect and the life can still collapse by a factor of ten through geometry and heat treatment. That is why heavy-duty backstop specifications must include alignment limits and hardness records, not only holding torque.

4. Engineering Solutions

• Reduce load per sprag. More sprags, longer contact lines and crowned profiles spread the same torque across a larger contact area; roller-type designs achieve the same effect by engaging several rollers simultaneously.
• Protect the surface. Specify 60–62 HRC races, controlled case depth (typically 1.5–2.5 mm for case-hardened races), superfinished contact bands (Ra ≤ 0.2 µm) and freedom from grinding burn verified by nital etch.
• Keep the oil clean. Filtration at 10 µm β-ratio ≥ 200 in gearbox-mounted duty removes the debris that indents raceways.
• Limit peak torque. A controlled braking ramp reduces transient peaks and therefore reduces the number of high-stress cycles — the dominant term in the damage sum.
• Inspect before failure. Borescope the race band at every major overhaul; a pitted band 8 mm wide is a scheduled replacement, a spalled band is an unplanned shutdown.

5. Series Selection and Verification

Documentation package to request: race and sprag material grade with hardness traverse (surface and core), case depth, retained austenite content, surface finish Ra of the contact band, nital-etch result for grinding burn, calculated contact stress at rated and peak torque, allowable misalignment, and a contact-pattern photograph from a dismantled sample unit. Any supplier who cannot produce a hardness traverse is guessing.

6. FAQ

Q: Is pitting a manufacturing defect or an application problem?
A: Usually an application problem expressed through the surface. Check sprag contact pattern first: even marking with pitting suggests material or lubrication; uneven marking points to geometry.

Q: Can pitting be prevented by better grease?
A: Only partially. Lubricant decides the surface stress concentration; it cannot compensate for a 30 percent hardness deficit or a 50 percent load concentration.

Q: How much does hardness really matter?
A: A few HRC points can halve contact-fatigue life. Hardness is the cheapest reliability feature in a backstop and the easiest one to falsify, so always request the traverse, not the claim.