Bowl lifting mechanism-kinematic analysis

Functioning of the 4 bar

KitchenAid sells to types of mixers: one where the bowl is fixed and the mixer translates up and down, and the other one where the mixer is fixed and the bowl translates. My mixer is of the latter kind as explained below. A fourbar mechanism (crank-slider) is used to move the silver bowl of the mixer up and down. The bowl is moved down to insert the ingredients with ease. The bowl is moved up so that the mixer is placed in the right position to operate. 

The mechanism is shown in the two videos on the right. The ground link is the frame of the mixer (link1). The crank is the shorter link (link2), represented by the yellow link in the Solidworks model. This link rotates from 0 to 180 degrees; at 180 degrees its motion is constrained by a spring which becomes fully compressed. The crank linkage is connected to an S shaped somber which the user rotates. The third link is represented by the red bar in the Solidworks model (link3), and it connects the crank to the slider attached to the bowl. The slider is not shown in the model but it is connected to link 3 by a nut and bolt connection. The slider translates vertically as link 2 rotates.

Although this mechanism is quite simple, it is very well designed. The linkages are sturdy and the overall design is very compact. Furthermore the majority of the mechanism is concealed inside the trunk of the kitchen aid. The mechanism accomplishes its intended task without compromises on the overall reliability and compactness of the mixer.

Kinematic analysis

Kinematic analysis of the 4 bar was performed in order to evaluate the effectiveness and the operation of the mechanism. The first plot on the right shows the variation of position of the slider as the crank rotates. The bowl moves approximately 6.5 cm vertically, as predicted by the figure. The results found through kinematic analysis match almost exactly the results measured on the mixer itself.

The mechanical advantage of the four bar was also calculated as a function of the input angle. The figure shows a spike between 120 and 150 degrees. This is probably a design decision, since at those angles the spring is almost fully compressed and a greater force is required to turn the crank. With this in mind, I tried turning the crank slowly, and it is definitely noticeable that at those angles a significantly smaller force is required. The mechanical advantage is always above 2 for the angles shown, which means that the user can apply lower forces to lift the bowl up. This only becomes useful if the bowl is very heavy, but it is still an important feature to have on this mechanism.

Overall, this simple fourbar crank-slider mechanism is a reliable and effective solution to the problem of translating the mixing bowl vertically. As seen by the kinematic analysis, it achieves a considerable translation length, that can easily be controlled by the linkage lengths. It also has a mechanical advantage at least greater than two, which saves the user some effort if the bowl is heavy.

Video 1 and 2: Fourbar crank-slider that moves bowl

Figure 2: Slider position (d) vs crank angle (theta2)

Figure 3: Mechanical advantage vs crank angle (theta2)