There is confusion about when crushes can be committed. Hopefully the information below will assist players.
These faults are generally called a crush - the mallet crushes the ball against the hoop upright.
Crushes are less frequent than people may think.
The Commentary on the Laws - the "Official Rulings on the Laws of Croquet" (ORLC) advises:
What Does This Mean in Practice?
There are both static and dynamic aspects to crushes.
For static situations you can readily define a range of safe angles that a ball can be hit through a hoop where it can slip through with no possiblity of being pressed against an upright.
In the plan diagram below the ball lies in the jaws of a hoop resting against one upright. The diagram is to scale with a 1/8" gap between the ball and the hoop.
The diagram shows that the ball can be played at an angle of up to ~20° out of the hoop without the possibility of a crush. The edge of the grey region lies on the tangent of the ball and the right hand hoop upright.
Consider however the situation below:
The stroke has to be played at no less angle than that defined by the perpendicular to the point of contact between the ball and the left upright in the diagram. Otherwise the ball will be forced into the left upright and be a crush.
At first inspection it appears that the ball will plough into the right upright - it does not have to be a crush though. The dynamic effects have to be considered.
As mentioned in the Commentary above, during a stroke the ball is in contact with mallet face for a brief time at the start of the impact and then springs away from the face. I have repeated Prof. Hall's measurements and confirm his observations. For a normal stroke the ball would 'stick' to the mallet face for ~ 0.25"; for a softer stroke a longer distance, 1cm = 0.4" is suggested.
The red line marks (approximately!) how far the ball needs to move before it makes contact with the right upright. Provided that this is greater than ~1cm and the mallet does not follow on then the stroke can be entirely legal.
Such strokes should be refereed since there is plenty of opportunity for things to go wrong. The likely faults would be a crush or multiple contacts between the mallet and the ball ('a double hit'), e.g. if the mallet catches up with the ball or the ball bounces back from the upright onto the mallet face.
Finally, crushes can happen in non-hoop running situations. For completeness an example is given below.
Here the yellow ball lies on the left hand wire. The red regions indicate where a crush would occur if the mallet is swung towards the centre of the ball. The green regions are 'safe' regions where the ball cannot be crushed - although near the upright the side of the mallet face would have to be used. The orange region is where it is also 'safe' to play although it would depend on the size of the mallet head as to whether it can get access through the uprights.
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