How will your beast open its mouth?
4
Ensuring reliability
One of the trickiest features of simple mechanisms is ensuring that they work reliably. Children find it frustrating when their mechanisms seem capricious and only work properly every now and then. It is important to ensure that there is sufficient clearance for moving parts. If a cam or a crank rubs against the casing this will cause problems. The worst case is that the cam or crank can't actually turn and the mechanism jams.
Often the simplest way to overcome clearance problems is to relocate the position of the shaft holding the cam or crank. This is usually simpler than changing the design of the cam or crank. Ideally clearance problems should be foreseen at the design stage, but sketching accurate cross sections which reveal the movement paths is not a simple exercise. It is often quicker and simpler to 'suck it and see' by means of rapid modelling.
A problem sometimes encountered with cams (and eccentrics) is that they do not grip onto the shaft holding them, so that instead of moving the follower they slip. This can be overcome by ensuring a tight fit. A tight fit is easier to achieve with wooden down than thin, stiff wire. A problem sometimes encountered with cranks operating the licking movement is that the tongue doesn't move exactly as required.
There are five features which may need adjusting: the position of the slot through which the tongue is moving; the length of the tongue; the location of the crank shaft in the side of the toy animal; the throw of the crank and any side to side movement of the tongue on the crank. You can explore this with a child by asking questions like these.
- Is the slot for the tongue wide enough?
- Is it in the right position? Does it need moving up or down?
- Is the tongue too short or too long?
- Is the crank hitting the insides anywhere?
- Is the crank moving the tongue the right distance? If not is it too much or too little?
- What will happen if you change the 'throw' of the crank?
- What will happen if you move the position of the shaft on the body?