A Ball Screw. General Catalog Torque Due to a Preload on the Ball Screw . • Torque Standard-Stock Precision Ball Screw Unfinished Shaft Ends. For THK Precision Ball Screws, a wide array of precision-ground screw shafts and ball [Structure and Features of Offset Preload Type Simple-Nut Ball Screw]. The Ball Screw is a high-efficiency feed screw with the ball making a rolling motion between the screw axis and the nut. Compared with a conventional sliding.
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Investigating the Terminal Strength of Ball Screw Shafts.. 團 Studying the specific value in the catalog by the motor manufacturer.) The inertial moment . Consult THK's entire Features and Types Ball Screw catalogue on DirectIndustry. Page: 1/8. CATALOG NoE ＊Product information is updated regularly on the THK website. . *When manufacturing Ball Screws of precision-grade accuracy C7 ( Ct7) with clearance GT or G1, the resulting clearance be partially negative.
I read your post but I'm a bit confused on the setup, perhaps you could clarify. I was under the assumption you were going to use a pair of TRBs face to face in the same housing like in a standard fixed end ball screw support unit. Do you plan on using one TRB on each end of the ball screw? From the way your describing it, it sounds like your going to squeeze the ball screw between two TRBs. If so, you're essentially running a fixed-fixed arrangement.
You will likely run into issues when the shaft grows under thermal expansion. If you put a spring washer on one side, when you have an axial load acting into that bearing the spring will compress and your whole shaft will shift.
I, personally haven't seen the arrangement I think you're describing. The only time I can recall someone using TRBs in a ball screw application was when they were used on the fixed side of a 50 ton press.
TechGuy , AM I read your post but I'm a bit confused on the setup, perhaps you could clarify. From the way your describing it, it sounds like your going to squeeze the ball screw between two TRBs I wasn't planning on using a spring, but perhaps thin compressible washer with a few mills of give. I think I will preheat the the ballscrew when I set up the mount to address thermal expansion. During Machine use, I could just cycle the ballscrew for 5 to 10 minutes to warm it up before actually machining a part.
I believe pre-startup cycling is standard practice for precision parts. Thermal expansion will also present an issue using radial bearings. Not using a fixed-fixed arrangement would introduce backlash. Your probably right. When I get it set up I will post my results. My concern is that the radials won't hold up to a heavy load.
My Z axis will probably have a lb milling head mounted. I agree with that the setup you're proposing you need to have them in a fixed-fixed arrangement to prevent the screw shaft from moving.
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You could technically preload them by offsetting their rings with spacers and then clamping the whole arrangement together, but there are better options out there. They are only really used in the floating end to carry the weight of the ball screw. They are typically press fit on the shaft and slip fit in the housing, so when the ball screw shaft expands they can float in the housing to allow the expansion.
The fixed ends typically uses a pair of preloaded angular contact bearings mounted face to face. The combined pair triplex and quad sets are also available for higher loading will handle your axial loads and prevent the screw shaft from moving under axial load.
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You can try looking at ACBBs with a higher contact angle 60 degrees these will give you more axial load capacity. I attached a jpeg with a screen shot from NSK's catalog with their heavy load supports. You can get their TAC series bearings which are the bearings inside these units separately too. If you know the loads acting on your ball screw you can easily determine what support would be adequate. What ball screw are you using?
What size bore bearing are you looking? TechGuy , PM I agree with that the setup you're proposing you need to have them in a fixed-fixed arrangement to prevent the screw shaft from moving. I would like to try going with TRB's first. If it fails to work easy then I will go with plan B, heavy duty angular radials. The Heavy duty blocks have a much larger height profile and TRB's have a very low profile. Thomson and Warner ballscrews are almost always rolled.
Star, and especially THK, offer a huge mix of rolled and ground. Just because it's a THK doesn't mean it's ground! NSK screws are predominantly ground. On site, use caution - USUALLY the seller will identify a ground ballscrew as such, while a rolled ballscrew is usually referred to as only a "ballscrew". Study them a bit.
One other method With that, let me touch on accuracy. Ballscrews usually have their own special model , which varies with the maker. Fortunately, the accuracy designation tends to follow a numerical format.
The numbers start at 0 and go upwards, usually by twos, so you can have a C1, C3, T7, etc. The lower the number, the better the accuracy. Ground ballscrews start at 0. Usually the designation is C0; sometimes you see P0. Some typical lead variations for different accuracy grades In other words, it is very difficult to create a 1 meter section of C0 ballscrew, but much simpler to execute the same over only 20 cm when a C5 tolerance is indicated.
The average lead variation for these screws is much smaller So you can see that ground screws can have terrific accuracy. They often come with a graph of their lead variation, created with a precision laser measurement device. Above C5, we enter the rolled ballscrew range. The very best precision rolled screws can reach C5, but these are also quite expensive, not much less than a ground screw.
Often, you'll see the C or P designation give way to a T designation, the T standing for "transport".
T-screws are often used in industry for actuating imprecise motion, like a flap on an aircraft, a gate valve, etc. But that doesn't mean they cannot be used for CNC! Just understand what you are dealing with. The majority of rolled screws you'll encounter will be T7 grade. Very quickly - don't get confused by repeatibility vs accuracy.
Even the crummiest ballscrews usually have excellent repeatibility, which is nothing more than having the ballnut repeat to a specific point. That's wonderful, but if the accuracy is poor, a high repeatibility won't help you if you need a part of a very specific dimension.
I'll use a ballscrew. It has a number of powerful advantages over ACME or any non-recirculating screwform. Hand in hand with the ballscrew is the supporting elements. To be effective, any leadscrew must be fixed axially to absorb thrust loads.
Yet it must be free to rotate so that your CNC machine actually moves. The answer I'm sure you've already guessed this is a radial bearing, commonly called a ball bearing. Before you run off and download some inline skate bearings, bear with me, we have more choices to deal with!
A C0 ballscrew is worthless if it is supported by a single, standard radial ball bearing. Such a setup has little ability to absorb thrust loads, and the entire ballscrew will move, along with the inner race of the bearing, under load. And it doesn't take much to move over 0. The ballscrew must be FIRMLY fixed in place on one end, usually the driven end, and to do this normally requires a pair of bearings, mounted back to back.
The ballscrew driven journal is physically clamped between two inner races, usually between a shoulder in the ballscrew, and a nut.
When the nut is tightened, the two inner races are loaded relative to each other. If we then firmly fix the OUTER races in a block or end plate, the ballscrew is now free to rotate relative to the plate, but will not move in an axial direction. This is best achieved by using what are known as angular contact bearings. In these bearings, the inner and outer races are tapered, meaning if they are loaded relative to each other, they will no longer move axially. Rather than try and describe such a setup with words, please refer to the attached print of a typical bearing block with two angular contact bearings.
Don't just glance at it if you do not understand the relationship, study it a bit and it will become clear. For simple setups, a pair of cheaper series of bearings will work fine.
For VERY light setups, two normal radial bearings, NOT angular contact, can be loaded in a similar fashion, but will handle nowhere near the axial loads that a true angular contact bearing can take. With the driven end fixed, why not REALLY fix the ballscrew in place by using another pair at the opposite end of the ballscrew?
Don't do that! The far end of the ballscrew must be free to float axially. This is due to temperature variations. As the ballscrew heats up, it will expand, and it must be allowed to do so, or binding and warping will result. The non-driven end of the ballscrew normally referred to as the simple support end has a simple journal turned or ground, usually to a length of perhaps 1. When installed, you'll want to create a modest gap between the squared-off ends of the thread, and the bearing inner race.
That gap will be taken up by expansion. It is possible to simply float the non-driven end of the ballscrew, but this will greatly limit the maximum speed that the ballscrew can be driven.
The simple support end of an axis is fairly easy to do, so it's best not to skip that step in your project. Even a T7 rolled screw will benefit greatly from a god set of fixed-end bearings.
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This is one area which beginners often skimp upon, and they then wonder why their 0. So how do we eliminate backlash? First, consider again the bearing set. If not fixed axially, you will create a backlash condition when you reverse the ballscrew under load. Assuming you have a good bearing set, all remaining backlash can then be attributed to the ballnut and its interface with the machine.
I am not going to go deeply into backlash and the ballnut. Very quickly, ground ballscrews are normally fitted with a zero-backlash nut. To do this, the manufacturer loads the nut with oversized balls; or, the ballnut ball tracks can be skewed slightly relative to the shaft tracks, thus loading the system.
If this is your situation, you are good to go. How it is done is not so important as the fact that it is common to mount a zero-backlash ballnut on a ground ballscrew.
If you are not sure if your ballnut is zero backlash, it can be tested with a very sensitive dial indicator say 0. Mount a handwheel on the screw, drive the ballnut, apply the indicator, then reverse. If the indicator needle responds to the slightest reversal of rotation, then you have zero or close to 0 backlash.
Rolled screws are tougher to deal with. Due to lead variation, if you load oversized balls, or skew the nut tracks, the system can bind as the nut travels into portions of the screw where the threads are a little closer together, or a little farther apart.
A good rolled ballscrew CAN be set up in a manner similar to ground, but far more common is the use of two nuts on the same ballscrew, mounted close together, with some form of powerful spring between them, usually belleville or stout wavy washers.
Manufacturers like Thomson sell assemblies that will do this, and the price is not excessive. If you can afford it, I recommend a manufactured, zero-backlash ballnut assembly for your rolled screw. What else? What pitch to use? With some rare exceptions, such as miniature, precision instrument ballscrews, most manufacturers' finest pitch is usually 4 or 5mm, or 0. Any of these is ideal. If the pitch is finer, smaller balls must be used, and this limits the load carrying capability.
Ballscrews lend themselves well to VERY coarse pitches; even a 16mm dia. These are not as desireable as a finer pitch. A coarse pitch will reduce the resolution of your system, and not transfer torque to linear motion as well as a finer pitch. Unfortunately, many of the surplus ground ballscrews available use a pretty coarse pitch.
Even if the price is very attractive, honestly, I recommend passing on any ballscrew with a pitch coarser than 5mm or 0. Metric or imperial?
It shouldn't matter. A decent control can handle metric or imperial, and output whatever product you want. Likewise, the direction of rotation is irrelevant. Lots to think about. Where are we now? We've discussed ground and rolled ballscrews, accuracies, and the need for a good bearing set.
Pitch too. Let's put this knowledge to use. What follows is pure opinion. If you disagree, then feel free to completely reject what I am about to say! Situation 1: "My goal is primarily 2D routing of hardwood. I want to build a BIG machine. I really cannot see doing any metal beyond a very occasional chunk of aluminum, and even then, the metal product can be somewhat crude so long as it is shaped correctly!
A pair of SKF series angular contact bearings loaded into a plate or shop block. The simple support end can go into a pillow block or other simple homemade bearing block.
Thomson factory zero-backlash ballnut, as routers tend to have large dimensions, and the loss of 1" of travel due to the length of a double ballnut usually isn't a problem. Repeatibility will be excellent. This will be a T7 accuracy system with zero backlash, and will chop wood all day long! The good news here is that while 0.
Problem: there's not too much space in a mini-mill for mounting the ballscrews.
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Again, I firmly believe zero backlash is very desireable, especially in a metal-cutting mill. Two nuts back to back, loaded, will do it, but that will knock about 2" off of the travel! Fitting the necessary bearings will be a challenge.
Solution B: Find a set of 12mm ground ballscrews, class C5 or better, with a zero-backlash ballnut. If they come with bearing blocks, by all means use them, and you will quickly have a killer axis, very accurate, and easier to retrofit than the rolled ballscrew.During Machine use, I could just cycle the ballscrew for 5 to 10 minutes to warm it up before actually machining a part.
A pair of SKF series angular contact bearings loaded into a plate or shop block.
You'll likely have to create a spacer in between the the two bearing rings if you want to properly control the amount of preload otherwise you run the risk of preloading the bearings too much which will affect heat generation and limit your speed.
T-screws are often used in industry for actuating imprecise motion, like a flap on an aircraft, a gate valve, etc. In this article, I will "scratch the surface", so to say, of ballscrew characteristics, and importantly, which type to use in your particular machine. The ball exits the nut at one end, enters the return tube, and is routed back to the other end of the nut, where it begins its journey anew.
What pitch to use? It's pretty easy with just a bit of experience. This page eBook is your guide to ball screws and details how they operate, their performance capabilities, different features available on them, and how to get the most out of them for industrial motion control.