Tonnage from Tie-Bar Stretch

δ=F.L/E.A

elongation=force x Original Length/(Young's modulus material x cross sectional area)

I always work in metric units (sorry to those who prefer conversion factors) take force in Newtons, Length in mm, Young's modulus in MPa, cross sectional area in mm^2 and you need elongation in mm. Rearrange the equation to solve for your unknown (F) then once solved convert this to what ever unit you want.

If you have multiple tie rods, you should be able to solve for each independently and then add the results together.

Ensure that your start (unloaded length) does not change, as if it does, E is no longer a constant and you have a plastic system on your hands, in which case you will need to examine the stress strain curve for the material.

Quite funny your suggestion as to scratch your left ear with the right hand.

He asks for the FORCE so that you use the elasticity limit of the bar material use a healthy safety coefficient and make the multiplication between stress and area. It is a straight forward approach.

But if the bars are fixed by threaded end the problem is totally different since the limit can be the thread not the bar itself! Then it is more complex the computation.

I found the answer,

the formula I used is as follows:

amount of stretch = Delta L

elasticity of the steel = E (30,000,000)

Tie bar area = R x R x 3.412 =A

spread across 4 tiebars

Devided by length of stretch = L

devide by 2000 (pounds per ton) = tons of clamp

thanks for all of your help

Erric8

And how much of stretch? What is the material for the bars? How is the fixation of the plates? How many cycles do you want your bars to hold?

If you treat the problem only skin deep then you get the risk to destroy the system.

I wouls suggest since the basic problem is not well known to you to use a service from a GOOD mechanical engineer able to analyse correctly the machine.

For an IMPORTANT problem DO NOT use blindly what a participant gives you as suggestion. Do not forget it is your job you risk not his.

Why do you ask this question?

If the tie bars are stretching on each machine cycle then you have significant opportunity for fatigue and subsequent catastrophic failure.

My understanding is that there should be no observable stretch in the tie bars in correct operation.

Tonnage should be calculation of observed hydraulic pressure, cylinder area and toggle mechanical advantage.

In a mechanical system there are 2 aspects to be considered:

- its own limit

- the loading

There is a difference to be made between "design" and "verification". As an engineer you know why.

What he asked for is the loading limit of the connection in order to determine how high can be applied load.

The bars will ALWAYS be stretched which ever the load will be. The acceptable stress level (considering fatigue) gives the highest acceptable load.

Stretching will consider the cylindrical part but will NOT take into consideration the thread loading which can be under circumstances the life limit for the system.

these machines are total electric, no hydraulic involved.

but thanks for your input on this issue

From my experience with a number of Hydraulic Injection Molding machines ( I take it this is what you are talking about) the normal strech of the tie bars is about .035". We normally test the strech over a 12" section and anticipate about .004-.006" per foot at maximum tonnage. We do this mainly to equalize the stretch on each tie bar so the machine has equal tonnage on all corners of the mold.

Hope this helps.

Sorry for being persistant on this but why do you want to know the stretch from the TOGGLE system on the tie bars?

Correct machine operation (from memory) has the toggle lock the tool closed (maybe with a small but fixed deflection) and then on injection of plastic the "locked" machine has to hold against the injection pressure projected over the area of the moulded part.

The utopian ideal would be an infinitesimally small clamp pressure at the point before injection.

If you are targeting a stretch in the tie bars BEFORE injection (That is only due to the toggle action) then when injection happens you then have the additive force on the tie bars risking significant machine damage.

Similar to the clamp loading on Automotive cylinder heads the internal combustion pressures do not exceed the clamp load, so no additional load is seen on the bolts. This limits the faitgue pulse on the bolts resulting in long life.

The Injection machine, hydraulic or toggle, pre stretches the tie bars so the injection pressure does not blow open the mold halves.

In an ideal world the machine would be designed to never allow the mold halves to open. In real life the machine is designed to lower the injection pressure near mold filling thereby allowing a smaller clamp to preform adequatly. Depending on the molding area it is almost always possible to open the molds with full injection pressures.

I've just been and had another look at the machines here. Maybe your machines are different, but here, the toggle system loads strain onto the tie bars and then the injection of material causes an additive strain on the tie bars.

Maybe the machines that you are looking at are different.

In the interests of advice to others that may read this thread in the distant future, I would thus suggest that readers should review the actual machine that they are working with to determine whether the forces are additive or otherwise.