Choosing the right Cylinder for Mold Core: Stroke

The stroke of a hydraulic cylinder is the distance covered by the piston and rod between the full-in and full-out positions.

It can be found as the difference between the maximum extended length of the hydraulic cylinder and its minimum retracted length. In Vega cylinders, stroke is part of the code and can be found on the nameplate.

Basically, in a cylinder catalogue, three values are given: Cylinder ‘block’ length, rod end protrusion from cylinder ‘block’, and stroke. Adding the first two you get the cylinder total length in the retracted position. Adding to both of them the third one, you get the maximum cylinder dimension, fully extended.

Note: from our website, you can download the drawings with rod full-in or full-out, as you wish.

Now we are going to review some issues and suggestions connected with the stroke you require.

•Short Strokes

We consider as such, all cylinders having a stroke below the ‘nominal’ value of about 100 mm. This is the case of most block cylinders.

Below this value, based on our case history, all problems which may rise in a cylinder are small or negligible entirely.

So, there will be no buckling.  Wear or leakages coming from side forces will very rarely appear.  Rod ‘shortening’ due to compression is normally negligible.  Speed and end-stroke shocks are low, and if you use a check valve, oil compression and consequent piston ‘bouncing back’ will be at their minimum. Only major design errors in the mold may induce cylinder problems in this case.

As we discussed in another post (Choosing the right Cylinder for Mold Core: Pushing Force) a hydraulic cylinder may be highly stressed while withstanding the plastic injection force from the molded part. Of course this force is, generally, exactly along its axis. During this phase we know the rod will be highly compressed and it is important to consider all the variables for avoiding defects on the molded part, such as burrs or ‘flashes’.

For example, the V260CF Hydraulic Locking Cylinder has been designed to face this problem and to ‘preload’ the rod for avoiding any defect. Looking at the catalogue (page F5) you can find the maximum compression value in mm which you can apply. Using these values and the correct pre-load flange, it is easy to avoid plastic ‘flashes’ or ‘burrs’.

Do you want to know how to apply Pre-Load on Vega V260CF Locking Cylinders? Watch this video.

•Long Strokes

Strokes much longer than 100 mm involve other challenges, especially inside a mold.

Apart from the issue of speed and kinetic energy of a large mass being moved (this can be solved with cushioning), there are others, connected with the average mold designer habits.

In fact, if it happens that any kind of strange mechanical parts, mostly in the form of brackets or wedges, are connected to the hydraulic cylinder rod. Often this connection is rigid, so it loads the rod with a bending motion. If the rod is full in, the forces contrasting this motion are low, and the flexural deflection of the rod is small. On the contrary, when full out, the rod is subject to high forces holding the bending motion, and the deflection at its free end, (depending of the free length cube) may be very important. Both these causes may lead to breakages, oil leakages or other problems.

For solving this problem, there are two favored solutions:

A)Use a floating joint between rod end and the part connected to it. This joint (if properly installed with the matching part) allows only axial forces to be transferred, thus cutting-off all bending effects on the rod.

Choosing Floating Joint

B)Use a spacer. This solution is adopted by Vega for the extra-long strokes described below.

•Extra-long Strokes

Sometimes customers ask for a special stroke, especially for the tie-rod cylinder V215CR. If it exceeds the maximum recommended one, Vega will likely advise to use an additional spacer. It is installed on the rod, and against the piston. When the rod is full out, it leans on the head inner face, so keeping the piston away from it. In this way the rod can be guided much better and is stiffer. Of course, the rod, cylinder tube, tie rods and consequently overall length, will be increased by the same value as the spacer length. Doing so, the stroke will remain the same as required by the customer.

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