Shear Collar

What does a shear collar really do? We replaced a shear collar yesterday thinking it might be bad, is was not (long story). Today we are numbering. Ok while not a true letterpress test we are packed with less packing and using less pressure to get the job done. I realized I don’t know what a shear collar really does. I assumed it was only an overload safety devise. Does it in some way assist in the control of pressure? Should I remove and clean it for any reason over time? The one we replaced appeared to have been there a while - at least for the 5 years we had the press. The old one was oxidized and worn but did not visible cracks or breaks. If this controls more than I knew about I would love a better understanding.

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if you’re loosing pressure, good chance its the shear collar. This is made of a special cast iron that will give way (the center part or inside of the ring), if the press has too much pressure. However the existing one may have a crack you can’t see and it sometimes happens that a faulty one is still in your tool tray because when that was changed no visible signs of crack were there. My advice, get a new one that you know is OK and try that. There is no time limit on these things and often times the problem of a sheared collar comes from the previous job you ran and didn’t notice.

To put it shortly. It’s a safety feature. The collar breaks instead of your press. So lay off the pressure as much as possible and your press will run forever. (With oil that is, black gold, Texas tea)

The shear collar doesn’t just break; it is designed to fracture and prevent catastrophic force from being applied to the platen or bed, and thus- the bearings that direct them towards each other.

It’s my understanding that…
The idea is that the collar is conical in form, loaded between the bed and the installation ring. Collar is fabricated in such a way that it will create resistance against enough force that the press can print- but not enough resistance that if the press were to push with more force than safe for the mechanism, the collar deforms or even cracks apart, creating enough ‘play’ in the system that the platen and bed cannot meet with potential force to damage the mechanism until the collar is replaced.

The collar could be deformed from repeated hard printing or even cracked in such a way that you cannot see it unless you inspect very, very closely, and still have done it’s job. It doesn’t need to be destroyed to indicate it is malformed.
I’m sure if you put calipers on it and calipers on a new collar you would not easily be able to tell a difference either.

I agree with the above posts, but my experience is these shear collars don’t often ‘shear’ 100%. They develop cracks, and your impression gets soft, and you require more impression, more packing. That’s a telltale sign right there.
It gives you time to get another shear collar ordered, but you should always have a new one on hand, in the package, so you know it’s new. On the 13 x 18, the shear pin is very different, and it breaks clean. You know it’s bad right away, which you may not know on the 10 x 15.

WOW - BIG THANKS (screaming thank you in all caps) First of all, thank you all for the great feed back. I think you all read my mind which is easy to do with all the blank space between my ears. I had read a lot of posts about impression control but don’t recall a mention of the shear collar described in this manner and with this degree of detail. The old one looked like a 100 year old brick. It looked like a pile of decayed metal but had no large cracks or breaks so I thought it was OK. I now - with your help - understand that this may be what was causing some of our issues. It most likely had gotten pounded down over time to the point we actually noticed something seemed different.

We replaced the shear collar and have noticed a difference. The packing math now works like I would expect and or impression is much more controlled. We do feel the collar was at least part of the problem. When we are talking about a distance or factor of 3-4 sheets of copy paper - 15-20 pts - it is not a lot but can change how things function. Our most recent job was packed to a 47pt with printed sheet and packing combined. The pressure was at the first ring past zero. I had a useable sheet on the first pull with a kiss impression like the days of old.