Insane Hydraulics

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My Thoughts on Application of Helical Rotary Actuators

I've wanted to devote a post to hydraulic rotary actuators for a very long time. Specifically - the helical spline rotary actuators, which use screws to translate linear motion of a piston into rotary. Like the ones manufactured by Helac or HKS. Please note that the following "rant" is my purely subjective opinion.

These actuators offer ridiculous amounts of torque in a very compact form factor. For example, the Helac's L30-215 boasts a whopping 24000 Nm at 3000 psi. That's like a large SUV sitting on a one-meter-long lever! If you look into the specs, and couple the generous torque to the declared radial and axial capacities of 8-plus tons and the straddle mount moment capacity of 17 ton-meter (however many SUVs this may be) and consider the fact that all of this is delivered in a 600mm-long and 310mm-wide package - I won't blame you if you think you've found the holy grail of mechanical torque generation.

Let us not forget about the fact that aside from getting a powerful "rotator", you also get a pivot point basically "for free", which makes designs a lot easier - and this is why a lot of (young) mechanical engineers may consider such actuators as the most perfect solution in the world for any "small motion range" rotating purpose, and begin including them in all of their their "revolutionary" designs.

This is the moment when yours truly comes in with a bucket of ice-cold water, to quench your helical rotary actuator lust, and (maybe) throw a wrench in your mechanical designs. In a good sense, of course.

The first thing that is "peculiar" about these actuators is the very thing that makes them move - the helical spline principle, which is great, but which also has a significantly low mechanical efficiency, and which is one of the reasons the holding torque of these actuators is about two times the nominal driving torque. This is never a problem when the actuator is driving a load, but it can become a problem when the load is driving the actuator - because any type of hydraulic dampening, like an over-center or a cross-port relief system, depends on how well your load-supporting arrangement translates load spikes into the respective pressure spikes, and these rotary actuators kind of suck at that (especially when compared with hydraulic cylinder based systems).

I guess what I am trying to say here is - if your load has a tendency to "push back", especially frequently and violently, you may want to swap a helical rotatory actuator for something else in your design if you want to build something that will last and work problem free.

Another thing that often misleads young engineers is the information they may encounter in tech brochures. For example, an engineer may look at the line of Helac's rotary actuators, and notice that they manufacture tilting couplers for construction attachments. If you read the product description paragraph in the service manual, you will find the following phrase:

" Thousands of contractors worldwide have come to rely on PowerTilt since 1990 for cleaning ditches, clearing land, digging beveled trenches, spreading riprap, and positioning brushcutters, mowers and hydraulic hammers. " (Highlighted by me).

One may think something like:

"...Hey! I heard that you shouldn't use a hydraulic hammer on a tilting coupler. I guess now I can. Hurray!"

Or even better:

"I am designing a mine scaler, which needs to be super compact and wield a heavy hammer on a long arm. I wanted to use hydraulic jacks for that, but I guess I can use them rotary actuators to hold the hammer now. Hurray!"

No. No hurray! I am sorry, but no.

The following quote is taken from the article on the versatility of tilting and rotating excavator couplers published at (direct link to the article here):

“Using a hammer with the PowerTilt is acceptable for intermittent use, unless it has been upsized specifically for a hammer application,” says Ron Hurst, product sales manager, Parker Hannifin Corp., Helac. “Some tilt-rotator brands do not recommend using hammers.”

Makes you think, doesn't it?

I am not saying that these actuators are bad. Not at all. They are great. And you will indeed find a lot of these actuators in tilting attachment couplers on excavators, which is a "tuff enough" kind of application as it is. It's just that you do need to be very careful about evaluating the catalog numbers, and the torque that your load will be able to induce on the actuator (in all axes, by the way), if you ever consider using one of these in your designs. Oversizing is definitely your best friend here, by the way. Oversizing and common sense.

If I were to sum this post up in a couple of phrases, I would say the following: