A washing machine balance problem
Email Washing Products Division
Moderators: Bill Whiten (University of Queensland) and Phil Broadbridge (University of Wollongong)
The problem
During the spin cycle, the inner bowl of a top opening washing machine can rotate about a vertical axis fixed to the base of an outer bowl, suspended from the 4 top corners of the washing machine body by 4 rods, each with a simple suspension system near their attachment to the outer bowl.
During spinning (at speeds of about 900 rpm), out of balance forces due to imperfect placement of wet clothes could cause considerable sideways and tilting motion of the rotating system.
To counteract this motion, a torus partially filled with liquid (usually water) is attached around the periphery and near the top of the inner bowl. The cross-section of the torus is rectangular and there are some radial baffles inside the torus to inhibit free flow of the liquid around the torus.
When the spinning inner bowl has reached its steady state speed, the motion is quite stable if the position, size and contents of the torus are suitably designed.
However, during development of a new washing machine, with different characteristics such as bowl height, diameter, mass or speed of rotation, it takes considerable time to redesign the torus. What is being sought, is a theoretical consideration of the balancing mechanism, to allow much quicker torus design. If possible it is necessary to achieve approximate balance for a wide range of uneven loads.
Some details
During spinning at low speeds in transient motion there is spectacular out of balance motion, and if this is excessive the outer bowl touches a switch in the machine body which will switch off power to the motor. It is the near steady state motion, rather than transient motion, that is the focus of this problem.
During transient motion the liquid in the torus moves to the outside of the torus in such a way as to substantially balance the uneven loading of the inner bowl. A suitably placed and designed torus allows balance of moments of forces acting on the axis of the inner bowl so the axis remains nearly vertical. For a particular load of clothes in a particular distribution, there is a design that will keep the axis stationary, and for similar loads the inner bowl axis will precess around an axis of rotation of the system.
It is known from experiment that a good design of the torus will allow quite acceptable motion of the machine system for a wide distribution of unbalanced loads. The sensitivity of the design parameters is not known.