salad spinner washing machine

Members: Mike Brady, Michael Murphy, and Rebecca Rosen


Our goal is to design an effective method of washing DRYING clothes that doesn’t require electricity and that can be constructed from local materials for a reasonable cost…

  • Effective: Getting clothes dry without a full-blown electric dryer requires a lot of work, time, or both. We want to create a system that can get clothes dry enough to be hang-dried in a reasonable time and in a damper climate.
  • Doesn’t Require Electricity: Washing and drying machines take energy to run. San Pablo has electricity, but is wanting to get off the grid, and needs to reduce their electricity usage in order to do so. Other developing communities don’t necessarily even have electricity to begin with, and it’s not even a good idea for people in developed areas to use as much electricity as they do. Alternatives include people power (hand crank, bike), water power, and possibly others.
  • Local Materials: ‘If you build it, they will come’ applies for sure, but if they can’t build it, what happens when it breaks? These aren’t likely to be Maytags. Using local materials means that locals will be able to construct the device, so they can both make more and repair the ones already constructed, both of which will bring in money.
  • Reasonable Cost: If the idea is to create a method in which people in developing regions can make money within their communities, it won’t work unless the communities can afford to buy the devices and have them repaired.

A Left Turn at Washing and into Drying:

Currently, the habit in San Pablo is to soak their clothes in soapy water, wring them out, dunk in clean water, then wring out again before hang-drying. The soaking method is easy, if time consuming, and we found many other easy ways to get clothes equally clean for cheap, so we switched to the less-analyzed problem of drying.

We came up with several ideas in four main categories for different dryers:

  • A pedal-powered dryer using pedals to spin-dry the clothes.
  • A top-like method, using a rope to start a tub spinning on an axle.
  • Hanging a bucket full of holes from a tree, rotate it to create tension in the supporting lines, then let go and let gravity spin it.
  • Put the clothes between two buckets, the inner one has holes, and apply pressure to squish the clothes between the bottoms of the two buckets. (Invented Later)

Cost Ease ofUse* Ease of
MaterialAvailability Maintenance Likelihood ofEffectiveness Time/InnovationFactor ^
Mult. Factor 9 7 9 10 8 10 3
Method Name Total
Pedal 6 9 3 6 4 10 3 345
Top 7 7 4 7 5 6 5 333
Tree bucket 10 8 9 9 10 6 10 487
Squish 10 10 9 9 10 8 8 515

* Ease of use has a lower than normal importance factor because the current method is fairly difficult.
^ Time/Innovation factor takes into account that this is a school project, with a finite time for development. Ideas that take less time to get a first prototype, take little time to make changes to, or that have more flexibility in their designs get high scores.

The Hanging 5-Gallon BucketSheer simplicity defined what should be our first prototype: hang a 5-gallon bucket drilled full of holes from a ‘tree,’ spin the bucket to generate tension, and then let go. We knew it would make an excellent sprinkler system…the question was would it dry clothes?


We videotaped each method that worked well enough to actually call a viable method. Later, we reviewed each video, writing down the frames in which each rotation completed.

Trial Rotations Spindown Time Avg RPM Max RPM Time Near Max Other?
2 Ropes 15 5.4s 179 225 2.7s N
4 Ropes 13 4.1s 199 225 1.9s N
6 Ropes 11 3.5s 199 257 1.9s N
4 Ropes + Weights 17 4.8s 220 257 2.8s N
2 Handle Ropes + Weights 27 7.4s 225 257 5.3s N

Rotations: Number of rotations from bucket release to the rope being fully unwound
Spindown Time: Time from bucket release to the rope being fully unwound
Avg RPM: Average of all computed RPMs
Max RPM: Highest computed RPM
Time Near Max: Time bucket was spinning at close to max RPMs
Other?: Any other notes about the data or method that bears consideration?

We were a little frustrated by the fact that we couldn’t get a taller anchor, which would have allowed for longer spindown times. In lab, we actually used a tool for lifting/moving car engines rather than a tree, because all the trees near the lab were too small to be sturdy, and we weren’t allowed to set off our sprinkler system on a walkway…In the end this turned out to be a good thing, as it allowed for some interesting modifications that would have been harder with a tree.


The problem was, even the fastest and longest of these methods was still leaving clothes dripping wet, so we needed a way to get the bucket going even faster. Looking at these trials, you can tell the tighter the ropes, the faster the bucket went, so we decided to pursue that idea. First we added bungee cords to the sides of the bucket, but this decreased the spin time so much that the clothes weren’t really drying any better. then we looked at using a rope or two tensioned by hand. This last method was extra promising once we figured out how, with a little skill, it could actually drive the rotation enough to keep it going.

Trial Rotations Spindown Time Avg. RPM Max RPM Time Near Max Other?
4 Ropes + Bungee 9 2.0s 280 360 0.9s N
Handle Rope + Bungee 18 3.7s 301 360 2.2s Instability near end caused slowdown
Handle Rope + 1 Bottom Rope 7 2.4s 198 257 1.3s N
Handle Rope + 2 Bottom Ropes 13 3.5s 232 257 2.7s Slowed by frame dragging…
2 Bottom Ropes Regeneration 8 3.1 169 200 0.9s Only a small, easy windup required


Currently, our best model is a version of the two-bottom-ropes design. The top rope, rather than being wound around the handle, is strung through the bucket as it was for the ‘2 Ropes’ method. It is also threaded through a pipe that sits betweent the two walls of the bucket to reinforce the bucket against the inward pull of the tightly wound ropes. Originally, we had been moving away from stringing the ropes through the bucket for two resons. The first was because hanging the bucket by the handle allowed for longer spindown times. The other was due to a jerk that resulted when the two ropes strung from opposite sides uncrossed and flared outwards, increasing the device’s rotational inertia and robbing it of speed. In this new design, we tied both the top and bottom ropes together at the point where this first cross/last uncross happens to gain the higher accelerations of the side rope designs without getting the jerk.

The Squish Method
In between coming up with new prototypes for our hanging 5-gallon bucket, the idea come up to stick our bucket full of holes inside one of the mud stove team’s slightly larger diameter buckets, and basically squish the clothes between the bottoms of the two buckets. There were some technical difficulties because our ‘practice load’ of laundry was fairly small, and the wide ‘rim’ of the buckets prevented the bottoms of the buckets from coming together enough to put pressure on the clothes. To counteract this, we cut the rim off the lid of our bucket to create a flat disk, threw some scrap I-beams into the bottom of the bigger bucket, and set the disk on top to make the ‘bottom’ of the bucket a little higher up. The results of our test were pretty amusing, but the clothes were about as dry as what was coming out of our spin dryer.

Dry Testing!

Speed measurements are good, but we wanted to do one better and actually measure how much water our various methods actually remove from clothing.
Sock Test:IMG_1678.JPG
For our first test, we added socks to our load of ‘practice laundry’. Uniform socks were soaked with the T- shirts, and weighed before and after spinning. the table is coded by the color of sock associated with the test

Sock Color Soaked Water Weight Water Weight After Drying Method Water Weight Lost to Drying Method Percent Water Lost
blue 2.8 oz 2.8 oz 0.0 oz 0 %
green 2.8 oz 1.4 oz 1.4 oz 50 %
orange 2.2 oz 1.4 oz 0.8 oz 36 %
purple 3.0 oz 2.4 oz 0.6 oz 20 %
pink 2.6 oz 1.2 oz 1.4 oz 54 %
white 3.0 oz 1.8 oz 1.2 oz 40 %

blue- control/hang dry for entire lab
green- regeneration method (two ropes top and bottom)
orange- bungees, 3 spins
purple- simple two-rope, 3 spins
pink- wringing by hand;
white- squish

As the table shows, the regeneration method was most effective at removing water (50% removed), after hand wringing (54% removed).

rafterdryer.pngT-Shirt Test:
After the close results between the regeneration and hand-wringing methods, we decided to go big before we went home. Instead of being hung from the engine hoist frame, the bucket was suspended from the rafters approximately 15′ off the ground, and anchored at the bottom using two pieces of PVC pipe, one attached between the legs of the tables, the other attached to the engine hoist. Three shirts were soaked together, and a single representative shirt was weighed before and after either being spun in our improved dryer, or wrung-out by hand.

Method Water Weight Soaked Water Weight After Drying Method Water Weight Lost to Drying Method Percent Water Lost
Hand Wringing 16.8 oz 10.2 oz 6.6 oz 39 %
Regeneration from the Rafters 16.8 oz 10.4 oz 6.4 oz 38 %

Based on this test, the two methods are very close with only a 1% difference between spinning (38% water lost), and hand wringing (39% water lost). This is very exciting, since it means our method gets clothes comparably dry to a very common human-powered method without adding much more equipment. Watch the super dryer in action!

IMG_1676.JPGThis was a really fun project to work on because it was simple to use and easy to tweak on the fly as new ideas popped into our heads. Our first prototype was crude in design, attach two ropes to a bucket, wind up, put in wet clothes, release, and see how much water was removed from the clothes with the resulting spin. This method proved to be far less effective than traditional wring drying, but taught us a lot about how the ropes affect the spin rates. From there, we began tweaking things willy-nilly, scrounging the lab for bits and pieces that might add a little bit of speed to our bucket, then going shopping if we came up with an idea that we couldn’t find pieces for. As we moved from rope, to bungees, to squishing the clothes, and back around to lots of rope, we even found a way to use the welding classes’ scraps. In the end, it was an idea that we initially abandoned that gave us our best results. The first tries at the regenration method, made as we were trying to improve on the bungee work, resulted in large knots that were annoying to untangle and didn’t spin anything, but once we returned to the idea and figured out that an equal tension on both ropes would keep that from happening, we were off. In the end, the regeneration method proved itself equal to hand wringing, a huge accomplishment for ten weeks, a plastic bucket, and a pile of rope. It was made all the sweeter by the fact that, though currently equal in physical strain to the hand wringing, a little more work should make spinning the bucket far easier.

Where the project is goingIMG_1669.JPG

The spin drier has been successful beyond our original expectations, and the amount of water removed with our latest test run is only about 1 percent less than the hand wringing method. These are very satisfying results, especially considering that we lost two weeks of the quarter when we changed projects. However, for future UNIV 392 students, there is definitely a lot of improvement possible with this design, as well as possible alternative designs we didn’t have time to build and test. The regeneration method was the most successful, and definitely more fun than wringing out the clothes by hand- but still a good workout, as Prof. Schwartz can attest to. A possible improvement that could make the spinning easier would be a foot pedal of some sort to regenerate the spin. This could possibly allow faster spins, since more force could be applied, as well as being less fatiguing for the operator. Another thing that needs to be worked out is how to go from the artificial environment of the lab to the real world. With the final regeneration design, the stability issues got bigger when both top attachments weren’t at the same height. This could be a problem when trying to hand the device from a tree, for example, and it is probably not the only difficulty in making such a location shift. Also, the biggest load we put in the bucket was three t-shirts and a sock. We have no idea how this method would respond to to bigger loads, with or without making a larger ‘bucket’. There is some very interesting and complex physics behind the deceptively simple hole-filled, spinning bucket, maybe even enough for a physics senior project as Prof. Schwartz pointed out. Physics and engineering inclined students could really have some fun analyzing and optimizing the design.

There are two completely different directions we stumbled across that next year’s students could also explore. They could go in the direction of bicycle power like Prof. Williams suggested, with some sort of stand to hold the bike stationary, and flip the axis of the output to allow vertical spinning. This would be even easier than spinning the bucket with the ropes, and would allow the bicycle to be used as a bike when not spinning laundry. Our squish method also holds promise, in that it is so ridiculously easy to use. It got a respectable amount of water out of the clothes, and we weren’t putting weight on it for that long. It’s possible to imagine of someone sat on the bucket while doing other work, the squish method might actually be competitive.

Whatever direction he next group goes, there is always more to discover, and who knows- maybe this design will one day be used in developing countries to save time and labor of the women and children who have the not-so-fun duty of daily laundry.

About us:

Michael Murphy

Is a senior in Earth and Soil Sciences, with a concentration in Land and Water Resources. His academic interests include mathematics, geology, climatology, physics, chemistry and materials science. He is considering a career in science and math education. Non -academic interests include mountain biking, motorcycle riding, and pizza.

Michael Brady

Agribusiness senior, with a concentration in marketing. Academic interests include surveying, economics, accounting, and international business. Has worked at Firestone Grill on and off for the past four years expoditing food and working in the kitchen. Hobbies include watching and playing sports,snowboarding, hiking and traveling.

Rebecca Rosen

Is a fifth-year Physics major with an Astronomy Minor. Her academic intrests include (predictably) astronomy and optics, and less obviously electronics, meterology, biology, and on rare occasions computer science. She teaches at the Cal Poly Observatory, and has a long history in backstage theater work. Her hobbies range from rock climbing and scuba diving, to drawing and writing stories.

Our Research:

external image laundrypod1.jpg
Here a group has taken what is essentially a salad spinner and made it into a washing machine for small loads that you fill using the sink. Plastic is probably a good choice for these guys, since it looks like they want to sell it to people in developed regions who are conditioned to like it, but unfortunately plastic isn’t something you can build with in most places.

external image sum2007_washing_greens.jpg

So the salad spinner game can go the other way…here a washing machine dries off lettuce at a farm. I remember last year there was a possible project that involved inventing a better way to pound and rinse a plant that was a skin irritant but very useful…is this still something with interest?

external image 180px-Wash1.jpg

These HSU guys simply attached an old bike to a broken washing machine. It sort of worked…they tended to distort the tumbler rather than spin it. Spinning things work better for longer when attached on both sides of any torque.

external image mit-washing-machine_d26z7_69.jpg
MIT is at it again. This time they’ve stuck an oil drum in front of a bike. Plastic pieces form the inner drum that actually spins. Also links on the page lead to a solar-powered motorized washer and two other pedal-powered designs. Video of Another Drum Design This time with a smaller drum as an inner rotator.

external image Wonderwash.jpgAnother store-bought version, this time one that works under pressure built up from hot water. The really interesting part was one of the comments, however. A guy used this washer in all sorts of conditions, and eventually figured out that it cleaned clothes just as well if he played with it like it was one of those ice-cream-makers-turned-toy. This is a possible way to deal with the ‘wash’ cycles, which on some machines aren’t simply a one-way spin. Toss it around for a while, use it like a balance ball, then load it onto a spinner to spin-dry.

external image smrnwm05.jpgThis one is really, really simple. It’s a plastic bucket and a toilet plunger. The bucket had a screw-on lid with a hole in it for the plunger handle, and the plunger had holes cut in it to make agitating the laundry easier. There is also a recipe for soap in this, though apparently plants and Borax don’t get along.

external image humanp1.gif
Here is a website detailing several different pedal- powered applications, as wells as some info about peak power out put, etc.


This is another website with some ideas about human powered washers. In addition to the ever-popular bike, there is a stairmaster version too.

external image mill-t.jpg
Maya Pedal in Guatemala designs and builds a variety of cool, useful machines using bicycles. They are developing a pedal-powered washer.