Shocking engineering

Ray Barnes of Hoerbiger-Origa explains how smooth, controlled stopping of moving loads can enhance machine performance and benefit the bottom line.

All moving parts in production and manufacturing machinery have to be stopped at some time.

Often there is a need for constant stopping and starting of one or more axes of a machine as it progresses through it production cycle.

And if the load is large, fast or even variable, then some design engineering effort has to go into this apparently simple requirement.

Let's start by stating that some axes of motion, say a comparatively slowly rotating carousal, can be allowed to stop in an uncontrolled manner.

Others can simply be run up against an end stop if they are incapable of doing damage through the sudden dissipation of their momentum.

But this still leaves us with lots of moving parts that cannot be left to their own devices, whose stopping must be controlled.

Now let's look at the advantages of controlled stopping.

Designing stopping arrangements is not just a necessary evil that consumes your valuable engineering resource.

Indeed the very opposite can be true with today's high speed, high performance machinery: by designing out shock loads and vibration (the greatest cause of premature machine failure), you can increase operating speeds and loads; improve system performance and reliability; reduce stress and wear leading to fewer breakdowns and less maintenance; reduce noise levels and achieve consistent finished product quality etc.

Put together these can have a major impact on productivity levels and production costs, so time spent on controlled stopping should be seen as an investment in your company's profitability.

After solid end stops, the next step up the stopping technology ladder is springs and rubber buffers.

These have a place, but they are not a universal panacea applicable in all situations.

They store the energy of the moving load, but then dissipate it in an uncontrolled way - possibly by sending the axis shooting backwards - indeed they can actually increase the shock loading experienced by the machine rather than reduce it.

But let it be said that they are cheap, and that is often a very great virtue, so use them if it is appropriate to do so.

One step up is the dashpot, a small piston that dissipates the momentum by squeezing oil though a small orifice from one chamber to another.

These are far more expensive than a simple spring, but don't rebound the load.

However, when you do a full analysis of a dashpot's performance, you see that the maximum shock load is virtually identical to that experienced with a simple solid end stop.

All told, then a dashpot is often not the best way to stop a load.

As an aside, pneumatic rod type cylinders often have built in end cushioning, a blind chamber into which air is driven as the piston advances to slow the piston in the last part of its travel.

It is worth noting that, counter-intuitively, the maximum load experienced here can be nearly equal to the maximum shock load that would be experienced without cushioning.

Further, the cushion is designed to stop only the mass of the cylinders piston and rod and not the mass it is driving.

It may handle a small mass at low speeds but at higher speeds and mass a cylinder cushion would simply not be able to dissipate the energy in a controlled way often resulting in the cushion being set to maximum.

This results in slowing the final travel of the cylinder substantially and adding to the machine cycle time.

Shock absorbers look like dashpots, but the two should not be confused.

A shock absorber works by converting the kinetic energy it absorbs into heat energy and dissipating this into the surrounding atmosphere.

It does this smoothly over the few milliseconds it takes to stop a machine axis or other load; thus shock load and vibration are avoided, with maximum reaction force being typically a quarter or a third than that of a dashpot.

This is achieved because instead of squeezing oil through one tiny orifice, the shock absorber effectively has a variable sized oil passage that starts large and gets steadily smaller as energy is absorbed.

In fact the oil passage consists of a series of orifices down the length of the inner tube through which the piston travels.

At first, all orifices are open giving a large flow at relatively low pressure; but as the piston advances it progressively closes off more and more holes so the pressure (which is in effect the shock load) remains almost constant.

In this way the shock absorber dissipates the energy of the moving load at a constant rate as the load is decelerated to zero velocity in the shortest distance and in the least amount of time.

Put another way, a shock absorber maintains a constant stopping force throughout the deceleration stroke, whereas a dashpot uses a pressure profile that varies from zero at the end to a high level maximum at the beginning.

All shock absorbers work on the principle as outlined, but there are variations on the basic idea so that the exact needs of any given application can be met.

Thus when sourcing a shock absorber, it is worth using a supplier who has a full range and can help you select the optimum one for each situation with the aid of computer sizing programs.

Obviously, size matters, so they come in a range of capacities miniature units of 10mm diameter to things capable of stopping a steaming train.

There are two main types of shock absorber.

The first is a self compensating model.

In plain English that simply means it is not adjustable and operates within a specified range of energy values.

These nonadjustable versions are better suited to applications where operating parameters are fixed and unlikely to change.

They are designed to give acceptable deceleration within their operating window of approximately 5:1 maximum to minimum weight ratio.

Adjustable versions have a simple adjustment mechanism for on-site fine-tuning between hard and soft action and are recommended for most deceleration applications especially where it is desirable to use the same model on a number of different installations.

Several mounting variations need to be accommodated, as does the option for a sprung return stroke.

They perform well under a wide window of conditions.

For example typically the weight range of these units is 150:1 meaning maximum weight capacity can be as much as 150 times greater than minimum weight capacity.

Shock absorbers from Hoerbiger-Origa are designed to give optimum performance in all applications.

They are manufactured using the highest quality materials and involve key design features to ensure long trouble free life, these include: a one-piece piston and rod made from high grade steel, hardened and heavy chrome plated for maximum wear resistance; an extra large diameter piston rod for superior strength; a proper "ball type" check valve for efficient oil flow and smooth operation; and extra long neck bearing for maximum support and side load capability; and a high pressure inner tube made from the highest grade carbon steel and heat treated for maximum strength.

We have seen that stopping machine motion is just as vital as starting it.

Gaining those extra few seconds by controlled deceleration of machine components can lead to improved throughput while eliminating shock load and vibration improves machine performance and reliability.

Getting it right will produce a better machine and ultimately have a positive impact on your financial viability.

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