>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 15 August 2005 19:53
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE] Lawn mower rock sensor
>
>
>> Needless to say, the blades get dull fast and rarely last 
>more then 2 
>> years and have to be re-sharpened every month or so.
>
>Count yourself lucky, I know a lady that's gone through three 
>sets of blades this year. (and a new quill :-(
>
>> I'd like to put some sort of sensor in that would disengage 
>the blades 
>> automatically when the blades hit something solid and I would like 
>> some suggestions from the group.
>
>My first thought for an engineering solution would be a shear 
>pin. Find a way to put something soft, cheap, and easily 
>repairable between the motor and the blades. (think of lead 
>for the key on the blade shaft) That way it limits the damage 
>to the rest of the parts. Or maybe you could make the blade 
>attachment squeeze between a couple of washers, sort of a 
>torque limited clutch.

I worked at an agricultural engineering company many years go. Lots of tractor driven farm machinery uses (or at least "used", not eactly up to date on modern stuff) shear bolts for protecting the transmission.  However, they are frustrating and time consuming to replace, I've seen numerous examples of farmers replacing the special shear bolt with a high tensile one...

Regards

Mike

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