Canadian Plastics

Taking (Mechanical) Advantage

Remember those mind numbing high-school physics classes where you stared at diagrams of boxes sliding on planes and billiard balls colliding? The basic math behind the equations of force and motion th...

October 1, 2008   By Jim Anderton, Technical Editor



Remember those mind numbing high-school physics classes where you stared at diagrams of boxes sliding on planes and billiard balls colliding? The basic math behind the equations of force and motion that govern the movement of simple objects was worked out a couple of hundred years ago, but if I’d known that they’d come into play years later, at press-side, I might have paid more attention in class.

In this case, I’m talking about the basic kinematics of mold action when it’s not making your parts, specifically opening and closing.

Closing a mold breaks down into the need to apply three forces. The first is to overcome static friction and get the mass of the moving platen moving; the second is the force required to overcome dynamic (sliding) friction and keep it travelling down toward its mate at the end of the tie bars; the third, of course, is the closing or clamp force. Unfortunately, clamp force is always orders of magnitude greater than the force required to move the platen, so it became the dominant design issue with machine hydraulics. . .until the toggle. This brilliant invention multiplies the force applied by anything from hydraulics to your hand in a simple, reliable mechanism. Here’s a diagram of a typical toggle and the math describing why it’s so brilliant:

What’s important to understand is that the critical factor is the angle “a”. At an angle of, say 30 degrees, the mechanical advantage of the toggle is 0.87 and 87 per cent, which delivers less force than you used to actuate the toggle. At 15 degrees, however, that “tangent” function comes into play, and the mechanical advantage goes to 1.86, almost double the input force. At five degrees, the mechanical advantage goes to 5.71! If you could precisely get a one-degree angle in the set-up, (unlikely in real systems with elasticity and lost motion from clearances as well as wear) mechanical advantage goes to 28.6!

Since you can easily deliver tons of force from a small hydraulic cylinder, add toggles and the force can easily multiply into the tens to hundreds of tons. There’s no fee lunch, however, as the price for the force multiplication is a tremendous sensitivity to the accuracy and precision of the small angles as well as link stretch and compression…not to mention lost motion from linkage bushing wear. Set correctly, the toggle snaps into place with a distinctive click and visibly smooth action…but in some systems, an eighth of a turn with the big wrench can unbalance a system enough to create serious issues with flash, or worse. Get much above 100 tons, and direct hydraulic clamping becomes a more repeatable, safer clamping technology.

I love toggles because they’re simple and they can last forever with not much more than regular greasing. Try that with control software!

CPL


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