One mechanism that can be used in a stair lift is a worm gear mechanism.

A worm gear is needed when there is a large speed reduction ratio is between crossed axis shafts which don't intersect. A worm gear drive consists of two elements: the driving element and the driven element. 

The worm drive has the worm wheel (the driven element) with a large diameter and a worm screw (the driving element) that meshes with teeth on the worm wheel.  As the mechanism is rotated the worm wheel rotates due to the worm having a screw like action.  "The size of the worm gear set is generally based on the centre distance between the worm and the worm wheel." (Beardmore, 2013)

 

 

 

(Beardmore, 2013)

Power is always transmitted from the worm screw to worm wheel in the mechanism. "Power cannot be transmitted from worm wheel to worm. This phenomenon is called self-locking. It is highly useful in many applications." (Sankararaj, 2013)

The number of teeth on the worm gear and the number starts on worm screw determine the velocity ratio. There is a decreases of the power transmission with an increase in velocity ratio.

There are disadvantages of using a worm drive in any system. Some of the factors related to use in a stair lift are as follows.

One of the main advantages is that the worm drive operates smoothly and silently. This mean the stair lifts will run with a gentle ride as it also has good meshing ability. The mechanism also is compact and takes up less space that some other mechanisms. This means the stair lift system can be compact and aesthetically pleasing. The worm drive can be used for reducing speed and increasing torque.

The major disadvantage of using a worm drive mechanism is that the worm gear materials are expensive. This can have a large effect of the cost of producing a stair lift for the manufacturer and also the price that the customer will have to pay for the product. Also, the worm drive has a high power loss and low transmission efficiency. Leading on from this the mechanism also produces a lot of heat so this would have to be taken into account when designing the stair lift especially in the mechanism casing.

 

From this research the evaluation matrix has been completed to reflect on these factors. It gives a clear evaluation of the mechanism when being used in the application of a stair lift.