A window cleaning robot stays on glass with a low-pressure fan or vacuum pump that holds its cups against the surface at 2,500-4,000 Pa. It loses grip when the cup seal breaks, the fan undervolts, or the machine overheats. Almost every fall traces back to those three causes, and two of them are maintenance problems.
How does the vacuum hold work in practice?
Two parts. A ring of soft cups presses against the glass to create a closed volume, and a fan pulls air out of that volume. The pressure difference does the rest. On a vertical window the machine is helped by friction against the pane; on a pitched skylight the same vacuum also has to resist shear, which is why a skylight job often needs a stronger hold than the same machine on a window.
Clean glass holds better than dirty glass. That is not a marketing line. A dust film gives air a path under the cup rim.
What are the real failure modes?
The table lists the four that show up most often in service returns.
| Failure | Symptom | First check |
|---|---|---|
| Cup seal break | slow slide downward | glass texture / dust film |
| Pump undervoltage | alarm at 2 min | battery, not motor |
| Overheat cutoff | stops after 30-40 min | ambient temp above 40 C |
| Edge sensor conflict | stalls at mullion | frame width vs cup size |
Why do machines stop after 30-40 minutes?
Thermal cutout. A fan drawing air through a sealed housing has limited cooling, and a roof in direct sun can push the intake air well above 40 C. The machine shuts down to protect the motor, which looks like a random stoppage to an owner watching from the ground.
Heat also thins the cup material. Rubber that holds 3,000 Pa at 25 C may only manage 2,400 Pa at 55 C. Running mid-afternoon on a hot roof is the hardest condition this class of machine sees.
Which Lingdu model suits which surface?
The Lingkong K3 is built for facade work where the pane is vertical and the hold problem is simpler. The Lingyun Y3 handles surfaces interrupted by frames and mullions, which is closer to a real skylight. The Lingfeng S1 goes after steeper roof glass where slip resistance matters more than speed. Details are in the Lingyun Y3 review and the reviews archive.
How do you test grip before trusting it?
Put the machine on the actual skylight with a tether attached, run one pass on the lowest pitch section, and watch the cup rims. If you see a bright line of air under any rim, stop. Test on the dirtiest part of the pane, not the cleanest, because that is where a real job starts.
What do the pascals on a spec sheet actually mean?
A pascal rating describes the pressure difference the fan can hold across the cup, measured in controlled conditions. It is not a weight rating and not a wind rating. A 2,800 Pa machine and a 3,500 Pa machine may behave identically on clean vertical glass and diverge sharply on a dusty 35 degree skylight, because what differs is how well the seal survives contamination.
Use the number as a coarse filter, not a decision. Below 2,500 Pa, expect near-flat surfaces only. Between 2,500 and 3,200 Pa, most commercial skylights are workable. Above 3,200 Pa, you are buying margin for dirt, pitch and wind, and paying for it in battery drain.
Suppliers who quote only weight and suction know that most buyers do not ask the follow-up question. Ask it anyway: hold at what pitch, on what surface, at what ambient temperature.
Why does the machine pause at frames and mullions?
Because the sensor logic is conservative. A machine that loses sight of glass cannot tell a 20 mm mullion from an edge with nothing beyond it, so it stops and reorients. On a window grid this is a minor annoyance. On a glass roof with frames every 1.2 m, it can turn a 60 minute job into a 90 minute one.
The fix is not more suction, it is a different chassis. Obstacle-crossing designs carry a cup array that keeps at least one cup on glass while another crosses the frame, so the machine never fully loses hold. That is a mechanical property, not a software setting, and it cannot be retrofitted.
If your skylight has more than four frames in the path, check the chassis before checking the fan.
What is the smallest number of cups the machine can hold on?
It depends on the design, but assume the machine needs at least two cups sharing the load to stay safely attached. Designs that rely on one cup at a moment of transition are the ones that drop when a rim fails. When you watch a machine work, note how often only a single cup is in contact; that is the moment of highest risk, and it tells you more about build quality than the spec sheet.
Does the number of cups change how the machine feels on a slope?
It changes how forgiving it is. A machine with more, smaller cups keeps continuous contact across uneven glass and tends to travel in a straighter line on a pitched skylight. A machine with fewer, larger cups holds harder on a wide clean pane but stops more often at frames. Neither is better in the abstract; roof geometry picks the winner.
Key Takeaways
- 2,500-4,000 Pa is the number that decides most jobs, ahead of any feature on the box.
- Plan around 40 C as a practical limit; beyond it the machine stops being the right tool.
- Maintenance routines beat spec sheet comparisons over a two-year horizon.
- Test on one pane with the safety device fitted before committing a full roof.
- Compare against a rope-access quote before buying, not after.