Below are answers to some commonly asked questions. Please feel free to contact us directly if you are unable to find what you are looking for.
Are roller screws multi-lead?
Roller screws are going to have multiple start threads. Depending on the diameter, you could have three, four, six starts of thread in any given diameter, so the manufacturing techniques allow for making those multiple thread starts within the screw.
Are the screws normally ground or machined?
For high precision designs, grinding is the normal process for both ball and roller screws. Thread rolling is common for lower cost, lower precision ball screws.
Are there any restrictions in the orientation of operation, horizontal versus vertical, for a roller screw?
There really aren’t. It becomes more a measure of what the force requirements are for the given orientation and how the load is supported. If you’re looking at a horizontal orientation that’s sliding along rails, you may have a higher inertia if you have a big, big load sitting on those rails, whereas if it’s a vertical application where you’re lifting and lowering a lot of tooling, you’re going to take into account that tooling weight. So really the orientation has no effect on the screw itself, but it could have considerations that are needed when you’re choosing the screw to make sure that you get the right screw for the application or package actuator.
Can roller screws be used in applications where high linear stroke speeds are required, for example a 24 inches per second moving a 1 ton load?
Yes, that would be a common application. In the integrated package design, you might see speeds that could get up to 40 inches per second. You might see forces that go up to 10,000-12,000 pounds. Now if you’re going 40 inches per second, we’re probably not going to be doing 12,000 pounds. We’re probably going to be doing more like 4000-5000 pounds at a maximum. But if you’re doing 12,000 pounds you might see a max speed of 5 inches per second. And then once you go to the conventional roller screw design where you’re adding back that external motor, then you really have a lot more flexibility on power range. You can get a really, really big motor if necessary. Depending on all the application characteristics, you can get a screw going pretty fast while also having a fairly high force, potentially upwards of 10,000 pounds or even 20,000 pounds of force. If you have a large enough motor, you could be going 20 or 30 or 40 inches per second.
Can roller screws go up to 50,000 or 60,000 pounds of linear motion?
Yes, Exlar produces conventional roller screw actuators from 5,000 pounds all the way up to 80,000 pounds. Check out our product selector (https://exlar.com/product/) to find the best actuator for your needs.
Can these designs be supplied into all industries – nuclear, petrochemical refineries for example?
Yes. Most packaged actuators are designed for general-purpose factory automation applications, but can be adapted for use in harsh or unique environments. The challenge, when you start getting into industries like a nuclear industry or certain oil and gas and refinery industries, is obtaining the certifications and ratings required. Is it possible to have packaged electromechanical actuators including roller screw actuators to be certified for those types of environments? The answer would be yes. As far as the Exlar® product line, we have some limited products that are available with some CSA hazardous ATEX/IECEx certifications for different parts of the world regarding hazardous location or explosion-proof type environment. So they are available. It really depends on the products and supplier.
Can you please provide a cost comparison between a ball screw and a roller screw actuator?
Cost comparison of a roller screw to a ball screw is really a difficult subject, mainly because we have to take into account the differences in the pieces that we are comparing. A roller screw is typically going to be competitive to a ball screw in regards to price because we can oftentimes use a roller screw that is smaller in size compared to its “equivalent” ball screw. This is because of the significant life advantage roller screws have. Therefore, if you are using a smaller frame size roller screw and comparing that to a larger size ball screw, with similar life expectancies, your pricing is going to be very similar. Now depending on what your needs are, if you are looking for something with much greater life, we’re not necessarily comparing an equal product. So you may have to buy two ball screws in comparison to one roller screw. If you look at that from a value standpoint, you may pay more for a similar frame size roller screw but you may have to buy two ball screws in the same period of time that you would have to buy that one roller screw.
How do you calculate the maximum duty cycle allowed vs the amount on current/force applied?
Below is the maximum-allowable duty cycle for your application given the percentage of input current over the continuous current rating:
For example: If your actuator has a continuous current rating of 10 A and a continuous force rating of 1000 lbf, this means it will take about 10 A to produce 1000 lbf of force, or 5 A to produce 500 lbf of force, and so on. What if you need to push more than 1000 lbf? In most cases, you would look at a stronger stator or a larger actuator. What if it’s only for a few seconds? Could you over-work the current actuator? Well the answer is yes, and calculating by how much isn’t too difficult.
Let’s say you need to push 1500 lbf. This would be equivalent to 1.5x the continuous current rating of 10 A. If you look below, the graph recommends no more than a 22% duty cycle in this case. This means you can run the actuator 22% of the time at 15 A without overheating. The other 78% of the time, it needs to be off/cooling.
How long can you run at peak current?
Not a simple question, nor a simple answer. In reality, so many things affect this (how the system is built and how well the actuator is able to dissipate heat, are there additional heat sinks, particles in the air, degree of vacuum, new starting temp each time? (i.e. doesn’t always start from cold, etc.). Therefore, accurate times and temperature are quite difficult to estimate.
For example: At peak current (2x Continuous), the allowable duty cycle is 4%. That doesn’t mean you can run for 4 hours straight as long as you have 96 hours of off time in between however. From experience, a good rule of thumb we’ve estimated is 30s to a minute of peak current run time. Try to keep it under that, and then of course allow it to cool for the other 96% of the time.
How does a roller screw compare to a hydraulic actuator of equal size and rate force?
That is going to depend on the application, but with equivalent specifications and characteristics, a roller screw actuator will typically be very similar in size to (sometimes slightly larger than) a comparable hydraulic cylinder. Hydraulics are always going to have their place in the market once you get beyond 100,000 lbs. of force, but anywhere an electromechanical roller screw actuator fits the bill, size will be very similar.
How does the back drive force compare against the ball screw?
The theoretical back drive force is going to be based on the lead of the screw, its efficiency and the amount of force being pushed against the screw. Typically with a ball screws are offered with a higher lead, and that higher lead is going to back drive a little bit easier. With a finer lead, just from physics you’re going to have a harder time to push that back. Comparing a ball screw to a roller screw with the same lead, the back drive force is going to be very similar depending on the efficiency.
How long until my specific actuator/application needs to be serviced/re-greased?
We are asked about re-lubrication intervals a lot. The reality is that there is no generic interval to re-lube actuators. It depends on so many things and every application and situation is different, it is nearly impossible to accurately calculate a re-lube interval per application. So instead, we have a rough guideline table (shown below) to give users an idea on when to start checking for old contaminated grease that needs to be replaced. However, since ambient temperature, heat dissipation, speed variation, particles in the air, etc. can vary so much from application to application, this is only a guideline. The actuator should be checked more frequently around the period this table suggests and once it is noticed that the grease is ready to be replaced (Dirty, contaminated / very dark, filled with particles / debris) – a re-lube interval can be determined.
Remember, grease needs to be cleaned out and replaced – don’t just insert more. (Except for FTX’s, those can handle 5-6 greasings before they need to be cleaned out)
|RMS Rotational Speed (rpm)||Recommended Grease Renewal Period (hours)|
If an actuator movement is repeatedly used in one spot, will this bring on premature wear?
In the case of an actuator that’s run in a single spot, you’re effectively using a singular process. When you’re using a pressing application, you’re going to end up hitting the same spot over and over and over again, so in turn your theoretical life calculation tends to be less the number of inches you can travel and more the number of times you can cycle to that same point over and over and over again. So that again plays a role in the life. We typically do have some derating factors that we use in order to provide our customers with an idea of what the best life they are going to get in pressing or a very short stroke type application.
What are the maximal operating temperature limits for a roller screw?
The maximum operating temperature limits for a roller screw are going to be based on the grease’s ability to handle that temperature, so unless you’re getting to the melting point of the raw material, you’re probably not going to have much trouble. As a system, however, the entire unit is going to be limited by the lowest temperature rated component – which would be the seals at 85°C. When we shift over to an integrated package design like the GSX, you’re going to have the permanent magnet motor thermal limitations. So in the case of most brushless motors, having an ambient temp of around 100° C at the case means about 130° C at the stator. Our motors are designed with a thermal switch to protect the motor and trip once it reaches an internal temperature of 130° C. To avoid this, we typically recommend a maximum of 85° C in an ambient environment. There is potential to go a bit higher, but it becomes very application-dependent once you get above 85° C ambient.
What conditions cause heavy contamination?
It depends on the environment. If a roller screw is packaged, contamination is typically going to be limited to ingress past a worn or damaged shaft seal. Shaft seals are a wear item and their life depends on many variables, including duty cycle, speed and cleanliness of the environment. Consequently, shaft seals are generally field-replaceable. Assuming the shaft seals are intact, there typically isn’t going to be contamination on a regular basis other than the screw lubrication migrating to other internal parts of the actuator.
What is the maintenance schedule life for a typical roller screw?
The maintenance schedule for any geared mechanical device, whether ball screw, roller screw, or gearhead, is going to be based on the amount of heat that is generated in the application, the amount of degradation of the grease, the type of grease being used, and the duty cycle. We provide some guidelines for our customers as starting points, but we recommend that for all new installations the lubrication be periodically inspected for presence and degradation as the best method for determining the right maintenance schedule for a given application. Having said that, we’ve seen repairs of units that have been in use for 15 years and when we’ve asked about grease renewal, they didn’t even realize that the unit could be serviced in the field. So we’ve had situations like that where they’ve gone for long periods of time with effectively no maintenance or no grease renewal. There are other applications that require grease renewal in very short intervals just due to the nature of the application.
What is the maximum linear stroke your design can be adapted for?
48 inches for traditional roller screws, 18 inches for inverted roller screws.
What keeps the output shaft from rotating?
On a conventional roller screw design package, there typically is an anti-rotation groove designed into the housing, and a tab designed into the nut that rides in the housing groove as the actuator extends and retracts. In regards to the inverted roller screw design, part of the installation or the application requirement is going to be having that shaft solidly mounted a machine coupling or tooling on the machine otherwise providing some sort of external anti-rotation device on that output shaft. There are other ways of using splines and different types of non-circular output shafts that can allow for different types of spline nuts that will provide anti-rotation, but typically you’re going to see that mounted on the machine.
When you calculate the screw life, how is lubrication considered?
The lubrication actually isn’t considered in screw life, and that’s mainly because we consider that a separate discussion. We separate maintenance from life. Life is assuming a consistent good lubrication over a period of time. The L10 life is typically what we offer, which is 90% of the time you’re going to get this life out of the screw. If a screw is not maintained at all over a period of time, is it possible it could last through its entire life? I would say yes, it’s possible. It’s probably unlikely in a lot of cases, but in certain cases you could have a situation where the screw would go through its expected life and meet the customer’s requirements without having grease renewal.