Study of an unbalanced washing machine

[MotoMike]

TL;DR

upon moving a washing machine from the floor to on top of a pedestal it seem to move around more than before.

the washer is a front loading type with axle of the drum going from the center of the front door to the back of the washer. the axle is close to horizontal. the pedestal is made for the washers and has a drawer in the bottom. the washer is firmly attached to the pedestal, essentially bolted together.

My thought was that the raising of the washer and bolting it to the pedestal which is about a foot tall, was increasing the mechanical advantage of the force exerted by the spinning drum if it has even a slight imbalance.

Thinking that raising the washer as described was lengthening the lever with the place it sits on the floor being the fulcrum.

Another thought was that I was raising the center of gravity and making the whole assembly more unstable.

I know it is probably a silly question but I appreciate your consideration.

 

[anorlunda]

I would say that all of those thoughts are correct. In part, they say the same thing with different words.

 

[berkeman]

I agree with @anorlunda that your thoughts are correct. Can you add a couple side braces to a nearby wall stud to re-stabilize the setup?

 

[jrmichler]

And also, by raising it, you changed the natural frequency. It is now closer to the running frequency during the spin cycle. Since the natural frequency is closer to the running frequency (revolutions per second), the vibration amplitude is larger.

Washer drums are always out of balance, so there is a rotating unbalance force vector that causes the whole machine to vibrate. The vibration amplitude is a function of the ratio of drum speed to the natural frequency, and also the system (washer, floor, house structure) damping.

Plus what @berkeman said. The braces will increase the natural frequency. If the new natural frequency is farther from the natural frequency, the washer will vibrate less.

 

[MotoMike]

thanks all for the time, information and thoughtful discussion. The vibration and movements in addition to seeming more once elevated, with time seemed to get worse. This precipitated me replacing the 3 drum shock absorbers and setting the washer on feet made of sorbothane. At present going through a testing period which after a couple days seems promising. Located on the concrete ground floor. braces may be considered if that does not solve the issue.

The changing of the natural frequency had not occurred to me and bears further pondering.
again, thanks.

 

[chemisttree]

I agree with @anorlunda that your thoughts are correct. Can you add a couple side braces to a nearby wall stud to re-stabilize the setup?

Recipe for disaster. Every time the machine runs the sound of the vibration and moaning of the electric motors will conduct to the walls and you will be inside your very own sound cabinet.

Try adding a lot of weight to the lower drawer?

 

[MotoMike]

that is an idea I'd not considered as well. thanks.

 

[marcusl]

Since the washer is worse now that it's on the pedestal, and you say the two are bolted together, the issue is with the pedestal.
1. Make sure the rubber pads are installed on the pedestal feet to keep them from skittering around.
2. Check that each foot is carrying the same load (a four legged stool with one short leg will wobble terribly). You can do this by putting a wrench on the height adjuster of each foot and checking that the same force is needed to budge it.

If that doesn't help and you are inclined to go further, then stiffening and dampening the pedestal is in order. The pedestal on my new Electrolux machine (it's the first time I've had a pedestal) has a steel frame with a sheet metal wrapper on 4-1/2 surfaces: top, sides and back. The bottom and front are open; the drawer, of course, provides no structural support to the front or anywhere else. Assuming that yours is similar, you might try the following:

3. Add 1/4" - 3/8" plywood panels to the frame inside of each surface including the bottom, screwing the edge of each wood panel to the frame every 2 - 3" or so. These form shear walls that stiffen the frame, eliminate any possibility of it wracking, and provide some damping since wood is lossy (especially compared to steel).
4. Shoot rigid spray foam (something like "Great Stuff) through small holes in the plywood to fill the space between wood and sheet metal. (Clean the metal surfaces with solvent first, and perhaps sand them as well, to provide a good bond.) This does three things:
a) Bonds the sheet steel to the plywood to keep the sheet from vibrating
b) Creates a very stiff double wall structure akin to a honeycombed wall
c) Adds damping due to lossiness of the foam.

This follows the general approach that both dampened and raised the resonance frequency of a large structure used for my low-temperature physics PhD research many years ago.

Some of the M.E.'s in this forum might be able to comment further.

 

[berkeman]

Every time the machine runs the sound of the vibration and moaning of the electric motors will conduct to the walls and you will be inside your very own sound cabinet.

Hmm, good point. I didn't think of that...