Hydraulic Pump Parade

Ever since I got the proof-of-concept mini-hydraulic system off the ground last year, I've been working to refine the elements, starting with the pump. As a brief recap of my previous findings: the traditional water pump I bought had a high flow rate, but not enough power to push open a syringe, even when over-volted. And since the motor is contained inside the sealed pump housing, there's no way to gear it down or otherwise modify it. The syringe pump I built for the system had adequate power, but was very slow (trying to run it too fast stalled the motor), wasted considerable energy overcoming its own internal friction, required check valves that reduced efficiency by trapping air bubbles, and sometimes leaked from the back of the syringe.

The syringe pump set up for maximum pressure testing. The small-diameter syringe is shown loaded into the pump; the large-diameter syringe and its cradle are beside it.

I decided I wanted to get some PSI measurements to better characterize my two pumps, and the difference between two variants of the syringe pump with different syringe diameters. I also wanted to build and test a third pump design: a peristaltic pump. This and the syringe pump both belong to the class of positive-displacement pumps, meaning that they only permit fluid to move one direction (inlet to outlet) and guarantee that a fixed volume of fluid is moved on each cycle - assuming, of course, that the motor does not stall and there are no other malfunctions. A third subtype of positive-displacement pump is the gear pump. I haven't tackled this one, mainly because the pump mechanicals contact the fluid, so the housing has to be sealed. I didn't feel like bothering with that yet.

Pump Design

A peristaltic pump moves fluid by squeezing it through a flexible tube. The tube is curled around the inside of the housing, and the motor connects to a rotary element that spins at the housing's center. This rotor has three or more rollers which contact the tube. Pockets of fluid are sealed between each pair of rollers and pushed along the tube's length as the rotor spins. Since the fluid remains contained in the tubing, there's no need to seal any part of the pump. The flow pulsates slightly, following the distinct "pockets" of fluid as they reach the outlet, but is more continuous than that of the syringe pump with its distinct intake/expulsion cycles.

Peristaltic pump version 1

There are existing 3d-printed peristaltic pump designs, some even open-source ... but I made my own so I could have modifiable design files in my preferred CAD program. (DesignSpark Mechanical/Spaceclaim doesn't seem to be widely popular among the 3d printing community, I'm afraid.) That way I can freely adjust the dimensions and motor mount design, add integrated bearings, etc. I designed my first peristaltic pump for standard aquarium tubing (6 mm outer diameter, 4 mm inner diameter) and the same salvaged stepper motor and gear assembly I'd used in the syringe pump. I figured I would get a better comparison by powering them both the same way

I went through two major design iterations to get a pump that worked well. The first version ended up not being quite tall enough for the 4 mm tubing - the tubing expanded so much when flattened that it tended to escape the rollers. So I made the second version deeper, but also reduced the diameter to decrease the size of each fluid pocket and the total length of tubing that must be filled during self-priming. I added guides to help hold the tubing down in the track, even when the front half of the housing wasn't on the pump (this feature isn't strictly necessary, but allows a view of the pump interior and fluid movement during testing) and integrated bearings for the drive shaft, since I notice it was sometimes binding against the housing in Version 1. I followed this bearing design [https://www.thingiverse.com/thing:4547652] (available in many variants around the web), but used 4.5 mm airsoft BBs instead of 6 mm. The outer half of each bearing is part of the pump housing, and the inner half is locked into place by inserting the BBs after printing. (I also tried a design with fully captive BBs that are inserted during a pause mid-print, but there wasn't enough clearance between the printer nozzle and the BBs, so they stuck to the nozzle and were dragged out of the race.)

The final major change between peristaltic pumps V1 and V2 was the type of tubing. I swapped the silicone aquarium tubing for latex tubing, which turned out to be much softer and easier to compress. It's available in a variety of diameters at relatively low cost, and it seems to reduce the rotational resistance of the pump considerably. I experimented with a smaller diameter of tubing, and it was easier to get fluid flow going with this size, but only at a reduced flow rate. I went ahead and optimized for the 4 mm tubing that was my original plan.

Peristaltic pump version 2, with geometry correction shim (black)

The last tweak that was necessary to get pump V2 to self-prime and move water with the 4 mm tubing was a correction of the pump geometry. I mistakenly set the bottom arc of the pump to match the outer diameter of the tube track (inner walls of housing), instead of the inner diameter of the tube track (inner walls of housing plus width of squashed tubing). This prevented the rollers in contact with the tubing from properly sealing it closed, because the tubing's resistance would instead push the rotor off-center, into the extra space created by the bottom arc's slightly too-large diameter. Instead of re-printing the pump housing, I corrected this issue with a shim. This prevented me from putting the pump's lid on; fortunately, because of the tubing guides I'd added, I didn't have to.

Tuning the roller diameter is also important. Too big and the motor stalls because the rotary resistance of the compressed tubing is too high. Too small, and no fluid can move because the rollers do not compress the tubing enough to create a seal. The range of workable diameters seems to be quite small; I had to print several sets of rollers to get their size dialed in.

Testing Methodology

I tested all my pumps by dropping a long piece of aquarium tubing from my upstairs window to the back patio, and measuring how far the pump could raise water up the tube. The maximum height to which a pump can lift fluid, measured from the top of the fluid in the reservoir to the top of the column raised by the pump, is called the head. This can be converted to pressure via the following formula:

pressure (PSI) = 0.433 * head (feet) * SG

SG is the specific gravity of the fluid, in this case water, whose SG is 1.

Pump testing on the back patio

I didn't attempt to measure flow rate. I only made the general observation that the submersible water pump is very fast, reaching its maximum head within a few seconds; the syringe pump, operating at a step rate that avoided stalling before maximum head could be reached, was agonizingly slow; and the peristaltic pump, operating at the maximum step rate of the motor (which produced both the best flow rate and the greatest head) was somewhere in between.

Data

Adafruit 3V submersible water pump

Syringe pump (9 mm inner diameter syringe), unknown stepper motor

Syringe pump (5 mm inner diameter syringe), unknown stepper motor

Peristaltic pump, 4 mm ID latex tube, 3 rollers, unknown stepper motor, step frequency 0.5 Hz

Conclusions

I like the peristaltic pump's balance of pressure and flow rate, and will probably try to use it in my next iteration of a hydraulic system. The large syringe pump excels at slow high-pressure operation and performs better at low voltages, however. It might be valuable in certain applications, if I could figure out how to avoid leakage.

I still want to experiment with different numbers of rollers in the peristaltic pump.

Until the next cycle,
Jenny