I've just received a baomain pneumatic cylinder from amazon (50 mm bore 100 mm stroke). When I tried to move it back and forth by hand, it wouldn't budge. Ive never used pneumatic cylinders for robotics before, but I'm fairly certain that they should be movable by hand. Does anyone know the awnser to this issue, or if it is even an issue?

  • $\begingroup$ What type of cylinder? Can you edit your question to add a link to the cylinder on Amazon? $\endgroup$ – sempaiscuba May 6 '19 at 23:21
  • $\begingroup$ did you take out the plugs from the ports? ..... upvote for not assuming that there is an issue $\endgroup$ – jsotola May 7 '19 at 1:08
  • $\begingroup$ this site shows red plugs in some of the ports .... baomain.com/cylinder#! $\endgroup$ – jsotola May 7 '19 at 1:17

Sometimes the seals can stick a bit if they've sat for a long time, but once you get past the initial pop they should move relatively freely. You're using a 50 mm bore, though, which is quite large, so there's a lot of surface contact for the seal.

Hook your cylinder up to an air compressor and try to move it. You should do this sooner rather than later so you can return it within your return window if the cylinder is defective.

As a side note, pneumatics are okay, but air is compressible. You should be able to calculate the force your actuator is going to output (acting surface area times air pressure), but the difficulty for control is that it's hard to quickly and accurately modulate the air pressure. It takes time to build up the air pressure and it also takes time to bleed the air pressure, because air is compressible.

You also have to take into account the shaft diameter when calculating acting surface area - the "extend" action uses the full bore diameter, but the "retract" action can only act on the surface area that is exposed, which is the area of the full bore diameter minus the area of the shaft diameter - $A = \pi \left(\left(r_{bore}\right)^2 - \left(r_{shaft}\right)^2\right)$.

Hydraulics are superior from a control aspect because you get near-instantaneous force modulation because you can change pressure so quickly in a hydraulic fluid, because it's not compressible. Tiny fluid volume changes result in immense pressure changes.


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