Open plain bushing bearing for use in linear motion

[rdcd]

Hi.

I was investigating on the internet for inexpensive bearings for a linear motion system. I found this plain bushing bearing (which do not use balls) from Thomson: FluoroNyliner® Bushing Bearings (equiavalent to Simplicity Linear Plain Bearing from Pacific Bearing) and would like to see comments for these bearings, comments on accuracy, play, durability, etc.

Regards.

[RICHARD ZASTROW]

Seem fine for light duty & what I call "low accuracy". I don't think they would be appropriate for metal machining, even though they have been used (not very successfully) in the past. Those machines were referred to as drill, tap and pad milling machines for non-ferrous metals.

Might want to check with the "wood butchers". LOL

Dick Z

[rdcd]

so richard, if i use the type with balls, the linear ball bearings, would you recomend it for "high accuracy" and some heavy works, i'm not planning to cut hard metals with this being my first DIY cnc, but maybe i would cut some aluminum... i was concerned about the price.

[packrat]

This is what they have to say in the McMaster-Carr catalog about that type of L/B. http://www.mcmaster.com/#linear-bearings/=6hotfr

cary



Linear Plain Bearings
For shafts, see pages 1058-1069. About Linear Plain Bearings


Linear plain bearings consist of a shell with a ceramic coating or a shell with a PTFE-based or nylon liner. They require no lubrication (except ceramic coated) and support heavier loads. When compared to linear ball bearings, they perform better in the presence of dirt, water, and vibration but don't perform as well with loads not directly over the bearing's center.
When selecting a linear plain bearing for your application, take into consideration the load, speed, force, the environment where the bearing will be used, and the position of the load in relation to the center of the bearing (mounting orientation).
The amount of friction between the shaft and a bearing directly correspond to one another. Linear plain bearings are suited to move higher loads at lower speeds because more force is required to move the load, and linear ball bearings are suited to move lighter loads at greater speeds because less force is required to move the load.
Dynamic Load Capacity— This value, expressed as PV (pressure x velocity), should not exceed 40,000 for ceramic-lined bearings or 20,000 (10,000 for 6396K, 6676K, 2570K, and 8639T) for other lined bearings. To check if a given bearing will handle your load and speed, calculate PV value for your application. Static load is the load on a bearing when it's not in motion. Velocity is the rate at which the load will travel; it should not exceed 1000 fpm for ceramic-lined bearings or 300 fpm (140 fpm for 6396K) for lined bearings.


Mounting Orientation— Fixed-alignment bearings do not accommodate shaft misalignment. Self-aligning bearings can adjust their load surface to accommodate minor shaft imperfections. Listed load capacity ratings are based on horizontal mounting position. Closed bearings can also be mounted vertically, on their side, and upside down without affecting load capacity. If open bearings are mounted on their side, the load capacity is 70% of listed capacity; if mounted vertically or upside down, load capacity is 40% of listed capacity.

No-Lube Linear Plain Bearings

Self-lubricating liners make these bearings virtually maintenance free. They are designed to be dimensionally compatible drop-in replacements for linear ball bearings; just slide or slip-fit a bearing into your housing and secure it with two external retaining rings (sold separately below). Choose from fixed-alignment and self-aligning bearings.
Closed Bearings— Use with end-supported shafts.
Open Bearings— Use with continuously supported shafts (also called shafts with support rail).
Fixed-Alignment Bearings


Frelon-Lined Closed Bearing

Frelon-Lined Open Bearing
Fixed-alignment bearings are designed for applications where shaft misalignment is unlikely.
Clearance is the space between the bearing and shaft. Bearings with tight clearance have an average 0.0005" clearance, extra clearance have an average 0.0015" clearance and provide additional leeway. They are primarily used on parallel-shaft applications. All are tight clearance except inch size bearings with an aluminum shell, which offers a choice of tight or extra clearance and the all PTFE-filled, which are extra clearance.
Frelon Lined— A liner made of Frelon (a chemically inert PTFE-filled composite) provides long wear, high load capacities, and increased strength. Temperature range is -400° to +400° F.
Bearings with an aluminum shell have an anodized Alloy 6061-T6 aluminum shell that provides good strength and corrosion resistance. Bearings with a stainless steel shell have a Type 316 stainless steel shell that provides superior corrosion resistance.
Use with shafts that have a hardness of at least Rockwell C35-C60 and an 8-12 rms micron finish. Note: Do not use with bare aluminum, chrome-plated steel, or 300 series stainless steel shafts.
For aluminum-shell inch sizes only, please specify tight clearance or extra clearance.
PTFE Lined— Have a chemical-resistant PTFE liner and no abrasive fillers, so they are safe for use on 300 series stainless steel and chrome-plated shafts. Shell is made of lightweight, nonmagnetic, electrically insulating fiberglass. Temperature range is -400° to +375° F.
Use with shafts that have a hardness of at least Rockwell B25 and an 8-16 rms micron finish.
Rulon Lined— A Type 304 stainless steel shell and an FDA- and USDA-compliant Rulon-641 liner provide excellent chemical and corrosion resistance. Temperature range is -400° to +385° F.
Use with shafts that have a hardness of at least Rockwell B25 and an 8-16 rms micron finish. Bearings have no abrasive fillers, so they are ideal for use with 300 series stainless steel and chrome-plated shafts.
All PTFE Filled— Ridges on the inside diameter move dirt out of the way as the bearing travels down the shaft. Liner and shell are made entirely of a PTFE-filled composite and are lightweight and corrosion resistant. Temperature range is -40° to +194° F.
Use with shafts that have a hardness of Rockwell C39-C70 and an 8-16 rms micron finish.

[RICHARD ZASTROW]

Add the machining forces to the dynamic load, then add a safety (fudge) factor.

The Cintimatics and Pratt & Whitney tapemates of the late 1960's and early 1970's had the ball bearing Thomson bushings. None of these machines were capable of anything but very light machining. Even drilling with anything above 3/4" in steel tended to make the table oscillate.

As usual, the work to be done will determine what the machine needs to look like.

Dick Z

[schbrownie]

Hi RDCD,

Can you tell us more about your plans are? I've used these types of bearing successfully in the past and had no problems with them. It all comes down to understanding the application and understanding what technology works best for each application type. Here are some of the questions to answer:

What size bed do you plan to build?
What material do you want to cut?
What is your required accuracry/repeatabililty requirements?
What will the duty cycle be?

Thanks

[harryn]

I did a lot of reading on that type of bearing. Buried deep in the data sheets is a small section on the X - Y spacing required to keep it from "hanging up".

Suppose that the distance between the bearings on the support rail is 2 ft. If the load is more than 1 ft from the rail, it has the risk of sort of oscillating as it moves down the rail. This seems to be the main design point that makes an application work or fail.

This might not seem like much of a limitation, but in a real world application for a hobby machine, it forces you to add considerable size to your design to have any motion length.

I also learned, from both reading and some simple testing, that there is no point in building anything, including a home hobby router, with an unsupproted rail, unless that rail is at least 2 inches in dia. A very telling test is to visit the local hardware store, and try to shake a 10 ft pipe of various diameters. You are getting close when you can't see it wiggle after a simple shake, and that continues right up to 2 inch. Its just amazing how flexible this stuff is in the real world.