Thread: Hinge pin mechanics

Gugi described the elegant action of the hinge/scale interaction very well in another thread as such.

Gugi's post;


There's one more nuance to this. Without taper you have a permanent stress on the scales/pivot in any position, which means that over time they deform a little and relieve the stress a little making the scales looser than what you want. So you have to retighten them.

With the taper at the wedge and the tang and flexible bowing out scales the extra stress (which causes the increased friction and scales staying tight) only during shaving. Provided you keep your razors closed when no shaving there is no deformation/loosening. It's a pretty simple design, but more complicated and sufficiently subtle that many novice makers/restorers do not understand its functional importance and go with a simpler design which works fine in the short term while the maker is still trying to part with it and during the first impressions by the customer.

And some (even big talkers with claims and aspirations to be among the best) are so confused they have been making scales with reversed taper on the wedge.

And then sometimes you see microfasteners advertised as a 'feature' to solve the problem of retightening the scales, but I look at it as poor way to try to compensate for a flawed design.

I noticed this elegant action shortly after starting to use a straight. The scales are utilized as spring as well as protection for the blade.

I copied this tapered tang feature on the first three razors that I made. My last three razors do not have this feature as the tang has parallel sides. I haven't really noticed a difference in the operation or function yet, but I'm going to start paying more attention. It makes sense that over time the creep in the scale material will give me more tensioning problems in the razors without the tapered tang feature.

Gugi's post reminded me of how cleaver this is. It avoids loosening of the hinge caused by creep as the pin tension is reduced when the razor is in it's closed position.

The other function of the wedge is purely from my perspective aesthetic as i find the gentle bow of the scales to complete the visual picture. They get a lot of discussion here on SRP. I have to think they deserve it because of their importance in the function and beauty of the scales, which is an important part of every folding razor.

Maybe it is just me, but I cannot help but think that a wedge, with the inherent flexing of the scales it causes, puts more stress on the scales than a simple spacer which does not cause the scales to flex at all. I can see how a spacer would allow for the scales to loosen over time (due to wear from opening and closing, etc.)

Am I missing something?

Originally Posted by RezDog

The other function of the wedge is purely from my perspective aesthetic as i find the gentle bow of the scales to complete the visual picture. They get a lot of discussion here on SRP. I have to think they deserve it because of their importance in the function and beauty of the scales, which is an important part of every folding razor.

Agreed, if your going to build a new razor, than using a spacer probebly makes no diff.
If your doing resto work,a wedge is mandatory IMO.

This may be right or wrong.

If the tang is flat with no tapers, then a spacer is OK

If the tang, at the pivot, tapers toward the tail you have a simple bevel

If the tang, at the pivot, tapers toward the tail (horizontal) and towards the heel (vertical) you have a compound bevel.

The spacer/wedge should match one of the three.

The taper(s) of the wedge should match the taper(s) of the tang at the pivot.

Originally Posted by Orville

Maybe it is just me, but I cannot help but think that a wedge, with the inherent flexing of the scales it causes, puts more stress on the scales than a simple spacer which does not cause the scales to flex at all. I can see how a spacer would allow for the scales to loosen over time (due to wear from opening and closing, etc.)

Am I missing something?

If you feel the tension on the blade as you move it through from closed to fully open, and watch the movement and flexing of the scales on a vintage blade it will show how that torsion changes as the blade moves. It is because of how the shape of the tang is moving through the scales causing different angles and the flexing keeps the movement smooth. If you then did a similar movement on a similar razor that has a spacer instead of a wedge the tension would increase and decrease throughout the range of motion as the contact at the pivot varies as the shape of the tang is not the same throughout the range of movement. Also when the razor is in the closed position the space would have it's greatest amount of tension. The old time designers had stuff fingered out.

Originally Posted by rolodave

This may be right or wrong.

If the tang is flat with no tapers, then a spacer is OK

If the tang, at the pivot, tapers toward the tail you have a simple bevel

If the tang, at the pivot, tapers toward the tail (horizontal) and towards the heel (vertical) you have a compound bevel.

The spacer/wedge should match one of the three.

The taper(s) of the wedge should match the taper(s) of the tang at the pivot.

Bolded seems to be what applies to my PRC Hydra.

Originally Posted by RezDog

If you feel the tension on the blade as you move it through from closed to fully open, and watch the movement and flexing of the scales on a vintage blade it will show how that torsion changes as the blade moves. It is because of how the shape of the tang is moving through the scales causing different angles and the flexing keeps the movement smooth. If you then did a similar movement on a similar razor that has a spacer instead of a wedge the tension would increase and decrease throughout the range of motion as the contact at the pivot varies as the shape of the tang is not the same throughout the range of movement. Also when the razor is in the closed position the space would have it's greatest amount of tension. The old time designers had stuff fingered out.

Yes, they did, but it is not the only method, obviously.

Originally Posted by Orville

Maybe it is just me, but I cannot help but think that a wedge, with the inherent flexing of the scales it causes, puts more stress on the scales than a simple spacer which does not cause the scales to flex at all. I can see how a spacer would allow for the scales to loosen over time (due to wear from opening and closing, etc.)

Am I missing something?

My only experience with a spacer in place of a wedge was with Hart scales which are one piece but functionally like a spacer. The only ill effect that I noticed was that the razor contacted the scales in the middle of the blade when closed and left scuff marks there. I'm told that Livi relieves his one piece scales in this area to prevent this contact point.

A tapered wedge flexes the scales out slightly creating clearance and I think having a little outward tension may keep a material that warps like wood more stable.

I think that the taper tang faction would work equally well with either a spacer or wedge. I could test this easily since I use micro fasteners in place of peened pins

For the record, I am asking out of genuine curiosity. The mechanics of both seem sound (to me, anyway), and I find the aesthetics of both to be pleasing. I will say that seeing a vintage blade with a spacer seems "off", but others may feel differently. I see the merits in both for both visual and mechanical functionality.