How does drum capacity affect spin efficiency?

Drum I occasionally receive questions asking about how the size of the drum affects spin efficiency. This is quite a technical question, and not something people are generally aware of. However, the size of the drum does make a difference due to the physics of it.

I know that in gearing, the smaller the drum pulley wheel is, the faster the revolutions of the drum, so presumably the larger the drum the faster the motor has to spin to achieve the same revolutions. I have done a little bit of research and found that the larger drums do appear to have a better spin rating compared with washing machines spinning at the same speed but with smaller drums. But there are anomalies, which means it’s not easy to make a direct correlation between the size of the drum and it is spin efficiency.


For example there is a Bosch 1400 spin washing machine with a 6kg drum which is rated B for spin efficiency, but there is also a Hotpoint 6 kg drum washing machine which spins at 1600 rpm, but despite having the same 6Kg drum, and spinning 200 rpm faster, it is also only rated the same B for spin efficiency. This shows there is more to spin efficiency than just the size of the drum and the spin speed.

The design of the drum, how many holes, what size holes, the pattern of the holes and also exactly how long it spins for must also have an effect. Within the time I spent researching washing machine stats I found that most washing machines with a 6 kg drum and 1400 spin speed were rated at B for spin efficiency.

Whereas most washing machines with 8 or 9 kg capacity drums spinning at the same 1400 rpm were rated A for spin efficiency. I don’t see this as conclusive proof because there are several anomalies but it does tend to give credence to the idea that the larger the drum the more centrifugal force and the greater the spin efficiency.

The following chart shows some of the ratings I observed –


MAKE SPIN SPEEDDRUM CAPACITY SPIN RATING
Indesit14005 KgB
Electrolux14005 KgB
Bosch14006 KgB
Hoover14006 KgA
Gorenje14006 KgB
Hotpoint16006 KgB
Hotpoint14008 KgB
Candy14008 KgA
LG14008 KgA
Hoover14008 KgA

Summary

At the end of the day the bigger drums tend to have the higher spin efficiency rating. However, there is probably less difference between A spin efficiency and B spin efficiency than most people are likely to assume. Here’s an article I wrote looking at the difference between tumble drying loads spun at different spin speeds and how much extra that is likely to cost – Washing machine spin speed efficiency figures and drying costs

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7 thoughts on “How does drum capacity affect spin efficiency?”

  1. You do not specifically mention it but it should be obvious that a larger-diameter drum does not need as high an RPM to acheive the same effectivness as a smaller-diamator drum spinning the same speed.

    If you do some research on commercial washing machines and extractors, you will see that they use a G-force rating. This is a simple algorythim based on RPM and drum-diamator.

    The sales-folks like to use spin-speed but it would be more realistic if the industry setteled on G-force rating which allows buyers to compare machines of different drum-diamators and spin-speeds with a common numerical factor.

    As drum-diametors get bigger, the G-forces go up exponentially. I would not be surprised that at some point, the G-forces of 1600 RPM and large drums start to breach some safety-factors. (Not to mention bearing-limits!)

    I expect that there is not much more to gain from spinning faster than about 1200 RPM with todays larger-diametor drums anyway.

  2. Thanks Bruce: For some reason manufacturers are producing much bigger drums but still making some of them spin at 1600 rpm with reduced clearance between the outer tub and inner drum.

  3. “the G-forces go up exponentially”

    Just linearly, but they do go up, so all other things being equal, the bigger the drum, the better.

    The centrifugal force (centripetal, if you’re being pedantic) does follow a square law, so doubling the speed quadruples the force. This is not trivial – clothes spun at 1200 rpm experience an eye-popping 386G, while 800 rpm gives a more relaxed 171G (these all assume a drum diameter of 48cm). I bought a 1600 spin machine because, nerd that I am, I did the maths and discovered that the extraction force would be nearly 700G, assuming it really did go at the advertised speed. Note that this is nearly twice as much as the force at 1200 rpm, so I maintain that speed is the important thing, although a big drum will help as well.

  4. “it is also only rated the same B for spin efficiency”

    I share your suspicion of the spin ratings. As with ‘efficency’ they don’t tell you anything very useful unless you know how they are arrived at.

    If one machine extends the spin for several minutes, it will presumably get a higher rating, but will leave your clothes more creased! The force involved, as I indicate above, is a straightforward calculation based on diameter and rpm, the more the merrier in both cases.

    I think A,B,C ratings are generally pretty useless – for instance, A-rated ‘low-energy’ lightbulbs produce an entirely different quality of light to their D-rated counterparts, and use far more energy in manufacture, are transported from China and create hazardous waste!

  5. An “A” rating will always be more desirable to consumers, even if the difference between A and B, in real terms is negligible, or even a decent difference but counteracted by other factors that could even make a B rating the best option (eg. extra stress and wear on the motor and other parts, £50 extra cost, extra creasing in clothes..). Unfortunately most people tend to make purchasing decisions based on emotional impulses rather than cold hard facts. Sales pitches are cleverly homed into that.

    I wonder how many people when faced with 2 products, one with an A rating and one with a B rating ever ask, what’s the actual difference between them? I’m sure most people’s thought processes are that A is obviously better than B so that’s what I should go for. It’s exactly the same with spin speeds, 1600 rpm is faster than 1400 so it must get laundry dryer and it must be better ..

  6. I’ve noticed with my newer, larger washing machines that my standalone spin dryer extracts less extra water than it did with the older, smaller ones despite having the same spin speed (both the same brand so little else will have changed much). I’ve calculated that my spin dryer with a 9in drum spinning at 2800rpm is moving clothes at a distance of 1319in/sec (((rpm*(diameter*PI))/60) – this is the same distance as the new one with an estimated 18in drum diameter spinning at 1400rpm, so the only perceptible benefit I now get from the spinner is being able to spin fabric for as long as I’d like rather than the pre-programmed duration on the washer. With an ever larger drum or higher spin speed on the machine, the spinner would be redundant.

    My take away from this is that if you’ve got a relatively slow-spinning washer (i.e. <1400rpm) or one with a narrow drum (diameter, not depth or weight limit is the important factor here), then a separate spinner is probably a worthwhile addition if you use a tumle dryer. Of course, the bearing loads on a large drum machine would be hideous – far higher than that on the spinner, even in the improbable scenario of a perfectly balanced drum, so the spinner would likely outlast them all anyway! Personally I set my washers to a low spin speed just so the clothes aren't dripping on removal, then finish them off in the spinner. That and not cramming the drum of the washer full means I've never had bearings fail yet.

  7. Andy Trigg (Whitegoodshelp)

    Makes sense Sector-9. Those little spinners are amazing. The slower a washing machine spins the longer it’s likely to last.

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