Chain Drive Stair Lift

·         The cornering ability is limited therefore chain drive is better suited to straight lifts only

·         Chain drive systems are also noisy and difficult to maintain.

Due to there being limited information available on chain driven stair lifts and only one known model (Brook’s Supreme) the conclusion was reached that a chain driven system is not a viable method to use. With the other two methods being better options to be considered 

Rack and Pinion design

The rack and pinion design is based around a rack mounted directly onto the bottom of a guide rail and a pinion gear that is connected the drive apparatus. The mated relationship between the rack and pinion is secured using several rollers to allow smooth movement along the guide rail and prevent any non linear motion.

FIG 1

Image taken from (http://www.google.com/patents/EP2452909B1?cl=en

As you can see from figure 1 this design uses a multitude of rollers, including a roller hidden by this image on the Y- Plane of the rack, to steady the drive apparatus and prevent any lateral movement. This design means that these followers bear a significant load , something that clearly can be achieved but will have to factor into our design process.

Another useful feature of this design is its ability to negotiate corners. The runner assemblies are mounted in such a way that they allow for a circular rotation , i.e the curve in the track,  whilst maintaining their resistance to the subjective loads. The two pinion gears are also mounted in a similar fashion  which allows for the curve of the track whilst maintaining their driven rotational motion. These features present some vital methods of overcoming some of the issues faced by our group from this project. However some of these assemblies seem very complex and may present some issues later in our design stage

FIG 3 (a stannah stair taken from https://www.google.co.uk/patents/US8087495?dq=stairlift&hl=en&sa=X&ei=6GqAVOXBIIzU7Aad8IGIAQ&ved=0CFoQ6wEwCDgK )           

The Reason for looking at this second design by stannah is to note the groove system mounted on the front of the rail to provide the stop/ start function upon reaching the top or bottom of the stairs . when the needle marked 67 on the diagram reaches the terminal marked 70, a mechanical signal is sent through the needle to the shaft marked 30, this shaft in turn turns a gear in which it is connected which will stop the stair lift instantly. This system will give us an interesting insight into a method of stopping our potential design.              

 

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.  

basic monorail stairlift sketch

CONCEPT

Tubular rack and pinion with stabilising rollers

SUB SYSTEMS

Rail/track

Control system

Braking method

Rack and pinion motion

Seat

Battery/charging mechanism

Seat with swivel capability

Adjustable erganomics

Fixing

Motor and gearing

Rollers/stabilizers

Matrix Criteria