Planetary Rolls Treatment Technology in Humanoid Robot Actuators

• 1
  What planetary rolls treatment technology actually changes
• 2
  Material selection and pre-treatment for roller screws
• 3
  Turning the screw, rollers, and nut raceway
• 4
  Heat treatment: hardness, case depth, and distortion
• 5
  Hard finishing: grinding, lapping, and surface finish
• 6
  Assembly, preload, and final measurement
• 7
  Which treatment step controls which failure mode

A planetary roller screw replaces sliding contact with rolling contact. The screw, six to twelve threaded rollers, and the nut raceway all roll against each other, so friction drops and the load spreads over many contact lines. That is why a humanoid robot knee or elbow actuator can push a rated load with a small motor and still hold position.

The catch is contact stress. Thread flanks touch over a very small ellipse, so a 40 mm diameter screw may see 2,000 to 3,000 MPa at the highest point. Surface defects, soft spots, or a 3 μm profile error all become fatigue origins under that stress.

Planetary rolls treatment technology is the sequence of operations that keeps those origins out: material selection, pre-treatment, turning, heat treatment, hard finishing, assembly, and measurement. Each step protects a property the next step cannot repair.

• 1
  Rolling, not slidingLower friction, higher efficiency, longer rated life.
• 2
  Small contact ellipseSurface and profile quality dominate fatigue life.
• 3
  Process order mattersA grinding error cannot be fixed by assembly.

Through-hardening bearing steel is the normal choice. SCM440 (JIS) and 42CrMo (GB) both reach 58–62 HRC after quench and temper, and they hold that hardness through the thread root where bending stress peaks. Stainless grades such as 440C or 17-4PH appear when corrosion resistance matters more than core toughness.

Pre-treatment starts before any cutting. Forging or rolling the bar breaks up segregation and gives a directional grain flow that follows the thread helix. Normalizing or spheroidize annealing then lowers hardness to roughly 180–220 HB so the bar can be turned without tearing.

Stress relief is the step people skip. A rough-machined screw carries residual stress from forging and from the first turning pass. If it goes straight to the furnace, that stress releases during quenching and the screw bends. A 600–650 °C relief cycle before hardening keeps straightness inside 0.05 mm per 300 mm.

Incoming material should be verified by mill certificate plus a hardness check on every bar. We also cut a test coupon from the same heat and run it through the full thermal cycle, so the furnace recipe is proven on the actual steel, not on a handbook value.

Turning produces the raw outline: screw blank, roller blanks, and the nut body with its internal bore. On a mill-turn center the screw blank can be roughed and the journal diameters finished in one setup, which keeps the thread axis concentric with the bearing seats.

Thread geometry is the hard part. The roller thread is usually a multi-start profile with a small lead, and the nut internal thread has to match the roller lead exactly. Pitch error accumulates over the nut length, so the tolerance budget on lead is tighter than on diameter.

Rough grinding comes next for any surface that will act as a datum. Journals, thrust faces, and the nut mounting face get ground to a reference diameter before hardening. Those datums are what the finishing operation will indicate from, so their roundness sets the ceiling on final accuracy.

Leave 0.3–0.5 mm of stock on thread flanks for the hard finishing pass. Less stock risks a soft skin after heat treatment; more stock means a long, hot grind that can burn the flank.

• 1
  Single setupMill-turn keeps thread axis and bearing journals concentric.
• 2
  Lead over diameterCumulative lead error drives nut backlash.
• 3
  Datum firstGround journals become the reference for hard finishing.

Quenching and tempering set the core strength. For a 40 mm screw, oil quench from 840–860 °C followed by two tempers at 550–580 °C gives a uniform 58–62 HRC through the section. The double temper is not optional on this geometry; it converts retained austenite and reduces the chance of delayed cracking.

Surface treatments change the wear picture. Gas nitriding adds a 0.2–0.4 mm case at 900–1,100 HV and leaves the part with a light oxide layer, which is useful on the nut raceway. Carburizing suits lower-carbon steels and gives a deeper case, but it moves the part more.

Distortion is the real cost center. A screw that warps 0.3 mm after quench needs straightening, and straightening adds stress back into the part. Press quenching or fixtured quenching holds straightness to about 0.1 mm per 300 mm, which is usually enough to leave grinding stock.

Every batch gets a hardness survey at three positions along the length plus a case-depth check on a sacrificial coupon. If one reading falls under 58 HRC, the batch is re-austenitized rather than shipped with a soft zone.

After hardening, the part is hard turned or ground. Grinding with a vitrified CBN wheel holds thread profile within ±0.005 mm and brings the flank to Ra 0.2–0.8 μm. That finish range is where the rolling contact fatigue life curve flattens out; going finer rarely pays back the cost.

Cooling and wheel dressing control burn. A burned flank shows as a temper-colored band, and under a 2,500 MPa contact stress that band becomes a spall in a few hundred thousand cycles. We dress every 8–12 parts and monitor spindle load rather than trusting the wheel to stay sharp.

Rollers are ground in a matched set. Diameter variation across the six to twelve rollers in one screw should stay within 1–2 μm. If one roller is 5 μm larger, it takes most of the load and fails first.

Lapping or gentle honing follows when the drawing calls for Ra 0.2 μm or better. It removes grinding marks without changing the profile. Polishing alone is not enough; it hides profile error without removing it.

• 1
  CBN grindingProfile ±0.005 mm, flank Ra 0.2–0.8 μm.
• 2
  Matched roller setsDiameter spread within 1–2 μm across the set.
• 3
  No burnTemper colors become fatigue origins.

Assembly sets preload. The nut, rollers, and screw are brought together with a shim or a ground spacer that controls axial clearance. A light preload of 1–3 percent of rated load removes backlash and keeps all rollers in contact, which is what a robot joint needs for repeatable positioning.

Preload is measured, not guessed. We rotate the assembly and record drag torque; a curve that rises smoothly with angle means the rollers are sharing load. A curve with steps means one roller is tight and the set needs to be re-sorted.

Run-in follows assembly. The screw is cycled under a light load for a few hundred strokes to seat the contact and settle the preload. After run-in the drag torque is checked again, because it usually drops 5–15 percent.

Final inspection covers lead error, backlash, drag torque, and surface finish on the flanks. Measurement reports go out with the parts when the drawing or the purchase order asks for them.