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I've recently bought the Mercedes-Benz Arocs model, which contains a compressor (i.e., a pneumatic pump which is operated by an L motor).

I was wondering whether you can ruin your pneumatic system and/or the motor by simply "overpumping" and thereby creating too much pressure? It doesn't look like there is some kind of "overpressure valve"...

If I just run the motor in pneumatic-pump mode and let it pump for a few minutes...

  • ...is there a chance to ruin the pneumatic hoses or even the cylinders?
  • ...is there a chance to ruin the motor?

If not, why not?

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  • I would guess the slight gaps in connections and such would provide enough overpressure protection for a LEGO-scale system.
    – chicks
    Commented Jul 10, 2016 at 13:03
  • Thank you for your answer! I'd still love to hear some more definitive information than "I would guess" :-)
    – D.R.
    Commented Jul 10, 2016 at 15:18
  • The Air Tech Claw Rig 8868 was definitely able at least once to overpressurize it's pneumatic system to the point of disconnecting a hose from a T-joint. Note however that this set was released in 1992, and the pneumatic components easily could have been updated since (for example with a safety valve).
    – zovits
    Commented Jul 10, 2016 at 22:27
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    In my experience, the pumps themselves have a very simple slipping mechanism which prevents overpressurizing. (I will do a little testing to get some numbers before writing an answer.)
    – PGmath
    Commented Jul 11, 2016 at 15:39
  • @zovits I have noticed that the old tubes seem to be a bit softer and have an ever-so-slightly smaller inner diameter, making them pop off a little be more easily than the newer ones.
    – PGmath
    Commented Jul 11, 2016 at 20:36

2 Answers 2

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No, it would be practically impossible to damage LEGO pneumatic elements by overpressurizing them.

It turns out that LEGO pneumatic pumps actually have a very simple "slip" mechanism that prevents overpressurization. Once the pressure reaches about 35-40psi (the exact threshold varies slightly pump by pump) the rubber plunger disk will give way and bend back, allowing a small amount of air to slip by and escape out the top of the pump.

Here is a GIF illustrating the effect, watch the rubber plunger disk bend back as I pump it:

enter image description here

Sorry the quality is so bad, it's hard getting under SE's 2MB limit. The number markings on the pressure gauge are multiples of 10, so it is slipping at around 37psi.

If you don't have a transparent pump or a pressure gauge it is still quite easy to tell that this is happening since the air escaping makes a fairly obvious pfft sound.

Some have also correctly pointed out that the tubes will pop off long before any damage is done to the components. While I have experienced this, the pump will usually hit its max pressure before any pipes pop off if they are attached securely (i.e. pushed as far as possible onto their connections).

Some pipes also tend to come off easier than others, in my experience the older ones along with the clear types tend to make looser connections. This is why I try to use the newer opaque tubes first since it can be a pain in the neck fixing and repressurizing when a connection blows off.

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    Great answer, nice GIF! Do you know if this slip-mechanism is also in place in the new, small pumps (used in combination with "compressors", e.g. the one used in the Mercedes-Benz I'm talking about in my original question)?
    – D.R.
    Commented Jul 11, 2016 at 19:39
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    @D.R. Yes, though it is a higher threshold, about 50-55 psi if I remember correctly. Unfortunately I don't have any working ones of those to test at the moment, they seem to have a lot of leaking problems in my experience.
    – PGmath
    Commented Jul 11, 2016 at 19:42
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You wrote you would like to hear an answer that is more definitive as a guess. I guess then we can accept Sariel as a definitive source, and he writes here:

There is no risk of anything blowing up. If you’re using Lego pumps, they will simply lack the power to keep pumping once a critical air pressure is reached. There is a risk, however, that your Lego pneumatic hoses may pop off the inlets.

This is exactly what I have personally experienced, so I would be quite confident in this being the truth.

Also, it would be a pretty bad idea from TLG to release an official set that can damage itself during normal operation, so even based on this alone I would say there is no risk of overpressure.

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  • Thanks - so, if the hoses do not pop off, the motor is going to stall, which is no problem either?
    – D.R.
    Commented Jul 11, 2016 at 6:40
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    I read this as if it's not the motor that would stall, but simply the pump won't push any more air into the hose. But in order to be sure I performed a test where I let my Unimog run it's compressor "dry" (without activating any switches or cylinders) for three minutes. After the first few seconds the hose between the pump and the front-rear switch filled and the pump started to sound a bit differently. There was no other observable change for the next three minutes and the system was fully functional after this trial.
    – zovits
    Commented Jul 11, 2016 at 16:34

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