6

I have just bought a double effect hand pump (a BETO CLD-024NAG) without knowing what to expect. I was very surprised to discover that the pump inflates the tire both when I push and when I pull. How does it work?

Also, the barrel of the pump is at the rear instead of the front (by front I mean where the valves and the manometer are) - is this related to the double effect?

pintoch
  • 163
  • 1
  • 6

1 Answers1

4

Pumps have an air chamber (normally cylindrical) that is compressed in order to force the air into the tyre. When the handle is pulled out, air behind the piston is forced out of the cylinder. In single action pumps, this air is vented into the atmosphere and lost (no big deal; there's plenty of it around!). On a double (or dual) action pump, this air is collected and forced through the valve into the tyre.

Single action pumps are mechanically simpler, and so smaller and lighter than equivalent double action pumps, but take roughly twice as many strokes to get to the same pressure. With a double action pump, you have to do work when pulling the pump handle, as well as when pushing, which is more effort per stroke (due to the pressure of the air already in the tyre, unlike in the single action pump, where you're working against atmospheric pressure which is much lower).

srank
  • 1,195
  • 4
  • 11
  • 23
  • Can you clarify more effort per stroke? Because I think it's the same effort, and no wasted effort expelling unused air. – andy256 Sep 30 '16 at 08:28
  • 2
    @andy256 With a single-action pump, pulling the handle out requires almost no effort because you're just moving the handle and not pressurizing any air. With a double-action pump, pulling the handle out is as hard as pushing it in, because you're pressurizing air and forcing it into the tyre. The "wasted effort expelling unused air" is practically zero: it's the same effort you make pumping the pump when it's not connected to your tire, for example. – David Richerby Sep 30 '16 at 08:37
  • @andy256, I've added a bit to explain my thinking - maybe I'm wrong, in which case I'll delete that bit of the answer. I think that each stroke (in and out) is more effort than the single action version, but overall less effort is wasted. – srank Sep 30 '16 at 08:38
  • @David and srank: Yep, the same effort both ways. But with a single action pump, it still takes effort to pull the handle back. Not much, but still work, so the total work is higher. – andy256 Sep 30 '16 at 08:41
  • @andy256 OK, yes. But not by any practically relevant amount. What was the point of bringing this up? – David Richerby Sep 30 '16 at 08:46
  • @David At first, to get clarity from srank, regarding effort per stroke, since what's written is incorrect, IMO. If the single action pump takes greater effort, even not by any practically relevant amount, it's still greater. Violates my innate sense of Physics. Always good to hear from you David :-) – andy256 Sep 30 '16 at 08:56
  • 2
    Note the "pull" stroke is not as efficient as the "push" stroke due to the longer high-pressure connection up the side of the pump. I couldn't find a good cutaway diagramme of a 2 way pump, will draw one if requested. – Criggie Sep 30 '16 at 09:17
  • @Criggie: I was also looking for a diagram… there has to be one somewehere on the internet, but I am not sure what keywords could dig it up. – pintoch Sep 30 '16 at 10:03
  • The "pull" stroke is working with a slightly smaller (in terms of effective square inches) cylinder (due to the presence of the shaft), and hence requires slightly less force than the push stroke. This effect is probably less than 10%, however. The difference in the length of the air connection for the pull stroke would be of little consequence in a well-designed pump with two check valves, though there would be some "wasted effort" on the very first stroke, as the air passages are pressurized. – Daniel R Hicks Sep 30 '16 at 12:18
  • 1
    Note that the reason for having a double-action pump is not to reduce effort, but to get the job done faster. Pumping up a tire with a shortie pump is a half-day affair. The double action allows you to accomplish the task in only 2-3 hours. – Daniel R Hicks Sep 30 '16 at 12:20