Is there a maximum practical parallel clamp length?

[Todd Brewer]

At what length do parallel clamps loose their parallelism or any other advantage? For example, Jet offers a 98" parallel clamp. Is it better than a pipe clamp at that length?

[HANK METZ]

Clamps that large are gonna be used in multiples, such as for panels and casework. At 30 lbs. each that's a lot of weight to shove around would be my concern.

- Beachside Hank
Improvise, adapt, overcome; the essence of true craftsmanship.

[Curt Harms]

I would think that above a certain length the bar is going to flex. I don't see how the jaws are going to remain parallel once the bar bows. Does the mechanism of a parallel clamp reduce or eliminate the tendency of the bar to bow under pressure? I don't know, don't have parallel clamps longer than 48" and use 24" 95% of the time.

[William C Rogers]

I borrow my neighbor's Jet 98 inch clamps. Holding them free horizontal they do flex as expected, however you can put them on straight. I am not sure how parallel they clamp, but seem ok. Nice when clamping something big, but unless there is a lot of use not sure if they are practical. I usually put pipe clamps together when needing extra length. You cannot get near as much pressure with the parallel clamps, but the larger foot is nice.

[ian maybury]

The engineering maths and mechanics say that (a) there will always be a deflection in a beam loaded as in a parallel clamp, and (b) - if my memory is correct - that the amount of this deflection up to the elastic limit is proportional to the fourth power of the length. Which in simple terms means that the deflection increases rapidly with increasing length, much faster than just in proportion to the length.

Practically speaking the jaws on doubled up pairs of long parallel clamps seem very unlikely to stay parallel or the beams straight unless the clamping force is very low. The closer the clamping force can be kept to the bar the less deflection it will produce, which is presumably why pipe and sash clamps have very short jaws. http://www.tooled-up.com/product/sealey-sash-clamp-1500mm/26013/ http://www.harborfreight.com/2-piece-3-4-quarter-inch-heavy-duty-cast-iron-pipe-clamp-31255.html

Blocks of wood placed in close to the bar on parallel clamps would have a similar effect - double sided tape would make is easier. If even at that there was a risk of tilting of the jaws pulling something out of square it might be best to use some sort of tilting pad under the jaw - say a block with a semicircular back face. Bessey do various jaw accessories and may have a ready made. More ideas here: http://www.toolreviews.ca/reviews_hand_tools/Irwin_XP600/Irwin_XP600.html ..

ian

[glenn bradley]

Somewhere around 48" I move to pipe clamps.

[Todd Brewer]

Thanks everyone. I was pretty sure they wouldn't be coast effective beyond a certain length.

[Ethan Melad]

i could be wrong, but wouldn't the depth of the bar have some impact on the deflection? i just got some older Jorgensen bar clamps - everything from 24" to 100+" - i guess maybe these don't really qualify as parallel clamps, but the bar is 1/4" steel at least 2" wide, which is wider than any jet or bessey parallel clamp ive seen.

[ian maybury]

The deeper the section, the stiffer the bar.

The stiffness increases in proportion to the cube of the depth = D x D X D. So it increases rapidly as you add depth.

The trouble is that it'll pretty quickly become heavy, and even at that the amount of deflection increases rapidly as above with length.

Various methods are used to try to help offset this - deep 'I' section solid beams, box sections, aluminium box sections etc

Beyond that it's down to limiting the length, and to keeping the jaws as short/close to the beam as possible so that the bending moment is reduced as much as possible. Think of the latter this way - you can clamp very nicely with a bit of string when the loading is in the line of the string.

It's when the loads are cantilevered some distance off at right angles to the line of the beam in the form of deep jaws (as on a deep F clamp, or a parallel clamp) that the requirement for lots of beam bending stiffness emerges - and why the length of clamps of this sort tends to be limited.

ian