I think I understand what you mean and agree it could be that. If an assembly line is running nuts down, they might want something like a loose (fast) fitting surface drive socket.
Koken Surface drive vs Flat drive sockets
- Thread starter Shelbylex
- Start date
Jack Ryan
Well-known member
No, not really.I think it will become obvious if you look at a picture of the profile.
The amount of material they remove during manufacturing determines how far a socket has to rotate before contacting a fastener.
With this type of socket, they remove a lot of material from the corners of the socket so that it engages much further back from the corners of the bolt. The benefit is that the socket is very unlikely to round the corners of a fastener. The consequence is that there is more empty space through which you must rotate before turning the fastener.
It's a kind of bitter consequence, too, because unless your ratchet has zero backdrag, you have to travel through the empty space once as you ratchet back, and then again as you start to turn. Not as big a deal if you hold the socket the entire time, but you can't always do that.
The profile of the driving surface is what is important. How is it necessary to place the reverse surface several degrees back further than is necessary to place the socket? Move those opposing surfaces closer together to remove the slop but retain their profile.
Of course, greater accuracy is then needed to place the socket removing the advantage cited by the manufacturer.
Back drag is something I have not previously seen mentioned here but what a pain the combination of slop and back drag would be.
Jack
mjdarg
Well-known member
Socket manufacturer's must have some tolerance clearance for the socket to fit on any fastener. A 13mm socket can't measure exactly 13mm or it might not fit on some fasteners. For surface drive sockets, since the contact is further from the corner of the hex, you get more angular movement for the same tolerance clearance around the hex. The spline drive sockets still contact more near the corners of the hex compared to surface drive.
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tolerance is the range of acceptability beyond the specified requirement. Clearance is what you mean. All sockets have designed in clearance beyond some condition of the target fastener. Tolerance is added to (or substracted from) the clearance.
To get a socket to fit the same on nuts that have tolerance, the tolerance of the inner profile of a socket must be nearly zero. No normal machined part tolerance would work. My guess is the best sockets out there are holding tenths
mjdarg
Well-known member
Yes, my mistake. This is what happens when I look at GJ at work.
Jack Ryan
Well-known member
You've missed my point entirely and I think we've all moved past the fact that a 13mm socket can't be exactly 13mm.
How does every other manufacturer achieve off corner drive without a large amount of backlash?
The backlash is not needed for off corner drive, but it is useful for the purpose listed.
Anyway, that's probably enough now.
Jack
mjdarg
Well-known member
Nope, not giving up that easily, and I don't think I missed your point, but just explained it badly. The whole point of this forum is to learn and discuss.
Say you need 0.002" clearance between your hex head and the socket interior wall. On a surface drive, that points that it rocks back and forth between are much closer than a normal flank drive socket. If the clearance at the arrows is the same 0.002", you will see a larger angular displacement on the surface drive. Project those arrows out the same distance at the normal flank drive socket pictured on the left and you will have more clearance. This is all a proportion of the distance from the center of the hex. If the surface drives contacts the hex half the distance from the center of the hex that a normal flank drive does, you will have twice the angular displacement given equal clearance gap.
Look at how close these marks are together? Sure they can tighten up the clearance to reduce the "slop" but its a side effect of the surface drive. Snap-On's FDX does the same thing, but the Surface Drive looks even more aggressive. I don't own either socket, I'm just going off visuals.
This picture says it all. If they wanted the contact to occur in the center of the flats, then there would not need to be (extra) rotational lash in the fitment. But the contact is made offset, and its directional.. so the lash is how the socket can alternate which side of the flat to make contact, according to the direction you wish to rotate it.
Jack Ryan
Well-known member
Apologies for the image, but just extent the lobe - or make a double lobe - as shown top left and the "slop" is gone.
The geometry of the lobe and the contact point on the fastener remains the same for both forward and backward movement but the travel is reduced.
I appreciate what you are saying, but I don't think any of that requires the slop and I think the FDX example you gave illustrates my point quite well - improved off corner performance with minimal slop.
Jack
