Pressure-threshold device with rack-n-pinion system?

Hello everyone,

I am ‘very’ new to the world of mechanical engineering, so please go easy! ha!

Attached to this post is a device that I wish to build for a university project. In brief, the device houses a one-way valve that will only open when a specific ‘pressure-threshold’ has been surpassed. The pressure-threshold can be set by extended the spring by a given length, thereby creating ‘pull’ on the bottom of the valve. I have built the chamber and valve assembly, however, I need help with regard to what components are required to ‘load’ the spring. I have already bought a relatively ‘heavy-duty’ servo motor. I am thinking that maybe the VEX linear slide pack, and the rack and pinion set (found in the advanced gear kit???) may do the trick.

Because I live in Australia, and have not yet found a place to buy VEX parts, the only place to gather knowledge about the VEX components is on this forum. If the rack and pinion system is the way to go, where mechanical/structural components would I need to properly mount the servo to the slide, and consequently, the slide to a fixed surface. Any suggestions, or improvements to the design is greatly appreciated!


IMT design.jpg


Far be it from me to deprive you of the fun you could have with a linear slide, but I don’t see why you need one for this device. Wouldn’t it be easier to put a drum on the output shaft of the servo and wrap the string from the spring around the drum? You could select the drum radius so that the angular range of the servo equates to the desired range of the location of the servo end of the spring.

If you use a rack-and-pinion mechanism, your adjustment range will be limited to the rotational range of the servo (in radians) times the radius of the pinion gear. Put another way, you need a pinion gear of approximately the same radius as the drum I suggest you consider.

All that said, I do not mean to discourage you from considering rack-and-pinion mechanisms in designing devices in general. Rack-and-pinion mechanisms have distinct advantages (e.g., rigidity) over the mechanism I suggested in many applications.

If you do want to build this with a linear slide from Vex, I point out that there’s a gear box (P/N: RACK-BRK-2PK) specifically for that application. The rack gear box can be seen on:
You could mount that gear box to whatever frame you already have, or build a frame for the entire device from Vex parts, such as those shown on:

Good Luck,

Here is an example of how to mount the rack-n-pinion.

This is a robot chassis that expands on user command (the kids call it expando-bot)

The mount is just a bent piece of standard vex metal.

Note you didn’t tell us how fast this needs to work or how much torque you are going to need.

That would be a big help …
RckNpin 1.jpg
RckNpin 3.jpg


that is a good idea about the drum. So, the drum would act as a wind or a ‘winch’? Initially, the device will need to be calibrated for use in an open-loop servo control circuit. This can be achieved by turning the ‘winch’ by a known angular displacement, and calculating how far the spring has stretched. Then, with a pressure sensor inside the chamber, one could derive a calibration factor of cm H2O per unit mm extension of the spring. Would the ‘servo-winch’ design be reliable over a long period of time? Or would it exhibit significant ‘creep’ in the tension applied to the string (requiring re-calibration frequently)?

Also, what materials would you suggest for the string, as it must not ‘creep’ over time.


the device is not required to be particularly fast, just accurate. The aim is to develop a software application that will adjust the tension on the spring (and therefore the ‘pressure-threshold’) in specific increments over time. For example, one incremental protocol may be 30cmH2O/min. Ideally, the device should have the capacity to impose a ~ 350-400 cmH2O threshold-pressure on the one-way valve.


Thanks again!

A winch is a fair term for it.

Is there some reason you couldn’t calibrate directly in units of cmH2O/degree?

I’ll get to the choice of “string” in a moment. Frankly, though, I’m more concerned about changes in the spring. In general, one way to reduce creep is to use things at far below their design loads. If I’m understanding the numbers in the drawing you attached correctly, the intended displacement of the spring is 0 to 4 mm. That’s not much (unless this is a very small spring) and should help manage spring creep. OTOH, that doesn’t provide much control range for the rack or winch.

Personally, I’m fond of steel cable. The problem I foresee is that you appear to have very little working range, so you’ll need to find a cable that will wrap around a very small drum, but won’t creep under the load. (OTOH, if you use a rack-and-pinion mechanism, you’ll have to deal with gear slop of the order of several percent of your intended range.)

As for attaching the cable to the valve and the servo motor why don’t you just use fishing line. Its VERY thin, strong, and lightweight. Under such a small strain I doubt that it would stretch at all (over a long period of time maybe). Now if you got a stand alone servo motor it should have come with mounting pieces like small things that fit on the shaft to attach to control lines. Here is a picture

The fishing line could be attached between the valve and those little holes on the parts that attach to the motor.

Here’s another way that I just thought up, you could use a small single acting pneumatic cylinder (I mean like pencil size) to pull the string in conjunction with a precise digital pressure regulator. Depending on just how “precise” it needs to be the regulator could be anywhere from $100 to $1000. That’s the only downside but it would be dead accurate some regulators can go down to really tiny increments and in such a small piston it would amount to a very tiny increase in force on the piston.

I’ve seen linear electric motors on Myth Busters, maybe that is what you should use? :slight_smile:

Oh yeah, for the earthquake machine? good episode.