What Is Weld Load (PD)?
Weld load, designated as PD in the four-ball test (ASTM D2596), is the applied load at which the four test balls weld together — meaning the lubricant film has completely failed and metal-to-metal contact causes the surfaces to fuse.
It represents the absolute limit of a grease's extreme pressure protection. Beyond this point, the grease cannot prevent catastrophic surface damage.
How the Test Works
The four-ball extreme pressure test is straightforward:
1. Four steel balls are arranged in a pyramid — one rotating on top of three stationary ones
2. The contact points are submerged in the test grease
3. The top ball rotates at 1,770 rpm
4. Load is applied in increasing steps, each lasting 10 seconds
5. After each step, the wear scar on the stationary balls is measured
PB tells you when the grease starts to struggle. PD tells you when it completely fails.
Why PD Matters for Harmonic Drives
Under normal operation, harmonic drives don't experience loads anywhere near the weld point. So why does PD matter?
Shock Events
Robots don't always operate under normal conditions:
During these events, the contact pressure between the flexspline teeth and wave generator can spike far above normal operating levels. For a fraction of a second, the grease film is all that stands between normal operation and permanent gear damage.
The Consequence of Failure
When gear surfaces weld — even microscopically — the damage is irreversible:
A single weld event can reduce the remaining life of a harmonic drive by 50-80%. And because these joints are sealed, the damage is invisible until performance degrades noticeably.
SmartC-HD: 315 kg vs 250 kg
In four-ball testing (ASTM D2596), SmartC-HD achieved a weld load of 315 kg. The industry benchmark tested at 250 kg — a 26% difference.
| Parameter | SmartC-HD | Industry Benchmark |
|---|---|---|
| Weld Load (PD) | **315 kg** | 250 kg |
| Difference | +26% | — |
This 26% margin means SmartC-HD can absorb significantly higher shock loads before the lubricant film fails. In practical terms, it provides a larger safety margin for unexpected events.
PB vs PD: Understanding Both
Both metrics appear in grease specifications, and they measure different things:
| Metric | What It Measures | Practical Meaning |
|---|---|---|
| PB (Last Non-Seizure Load) | Load before any seizure | Normal operating protection limit |
| PD (Weld Load) | Load causing complete welding | Absolute failure point |
Some greases have high PB but moderate PD, or vice versa. The ideal grease for robot joints should have strong performance in both:
Selecting Grease Based on EP Performance
When evaluating grease for robot joints, consider both PB and PD in context:
| Application | PB Priority | PD Priority |
|---|---|---|
| Cobot wrist joints | High — smooth continuous operation | Moderate — limited shock exposure |
| Humanoid hip/knee | High | Very High — walking, stumbling, jumping |
| Robot dog legs | Very High | Very High — dynamic gaits, impacts |
| Industrial robot base | Moderate | High — heavy payload handling |
| Dexterous hands | Moderate — low loads | Low — minimal shock risk |
For applications with high shock risk, PD should be weighted heavily in grease selection.
Conclusion
Weld load (PD) is the grease specification that protects your most expensive components during their most vulnerable moments. It's not about everyday operation — it's about surviving the events that would otherwise destroy a precision gear set.
SmartC-HD's 315 kg weld load provides a 26% larger safety margin than the industry benchmark, giving robot OEMs confidence that their joints can handle real-world conditions — not just lab conditions.
Request a sample and test SmartC-HD on your own hardware.