Sizing Bushings and Pins

[Corm]

I had this question while reading the thread about the HF metal cutting circular saw, but didn't want to hi-jack that thread.

Is there a 'rull of thumb', or a guide, one can use to determine the size and length of bushings and pins for a project? While reading the thread mentioned above, Larry included some nice pictures of his bushings and pins. How did you arrive at the size you used?

I've got a couple of projects that I want to do over the winter that will have bushings and pins, and am just curious what you experienced folks use. I've got a couple of lathes and a BP mill. I've made some bushings and pins in the past, using whatever material I happened to find in my bucket of small pieces left over from other projects. I just went with whatever size seemed to be right, not knowing if there is a guide of some sort.

Thanks in advance

Corm

 

[J_J]

Brass bushings will work in some situations for light weight use. DOM tubing with a grease fitting for heavy duty use.

 

[Shmudda]

Corm,

The size of the pin and bushing all depend on the load its designed to see. Pins are usually operated and loaded in shear, so those loads are really easy to figure out.

As for bushings, the rule of thumb is: If you have something that is turning at very high loads, at a very low RPM the the bronze/brass bushing is the way to go. If you have something that is turning at faster speeds and lighter loads then ball, spherical, taper roller or needle bearings are the way. You must remember that everything has its purpose based on its design.

I work in a steel mill, and there are many, many applications where the pin and bronze or brass bushing is the prefered design based on the application and the rule of thumb I stated above.

Craig

 

[Corm]

Thanks J_J, & Shmudda, for your replies. Much appreciated!

Corm

 

[Brad_Blazer]

My rule of thumb is to look at similar applications and copy what they are doing. Check out your loader pivot pins, 3ph pins, etc. to get an idea of what works. Pay attention to geometry such as pins captured in double shear (e.g. typical top link)vs. pins in single shear (e.g. typical lower lift arm).

I like the 1" pin and 1/4" wall bushing combination like Larry uses because it's strong enough for most applications and the sizes are easy to find.

 

[JustGary]

Corm -

Use the strength of the material and the area of the part as your guide. A 1" ID bushing with a 1/4" wall will have a cross-sectional area of
(1.5/2)^2 * PI = 1.76 sq. in. (the outside)
minus (1/2)^2 * PI = .78 sq. in. (the inside hole)
= .98 sq. in. (let's just use 1 sq. in.)

Common A36 steel has a strength of 36,000 PSI, so our 1 sq. in. of bushing material has a strength of 36,000 PSI * 1 sq. in. = 36,000 Pounds. A higher grade steel (such as A519) will quickly get you up to about 50,000 or 70,000 pounds for your bushing. For most tractor work, this is plenty, even including a 50% margin for safety.

As for the pin, the 1" diameter gives you (1/2)^2 * PI = .78 sq. in. of cross-section, so A36 steel gives you about 28,000 pounds of strength. That's still well within 50% for a 3" cylinder at 3000 PSI provided that your pin hits a bushing on either side of the cylinder rod (in other words, the pin has to shear in two places at once). Again, stepping up to higher grade steel in your pins will provide more strength.

After several good inputs, I have decided to use mild steel pins for my root grapple, since I can change them a lot easier than the bushings (which are A519 DOM tube). I want softer metal in the pins so they wear first, and they won't shear off.

As for the length, use what will conveniently fit your application.

Also, figure about 1000 pounds per inch for welds, worst case. Your bushings won't do any good if you don't weld them on right.

Regards,

- Just Gary

 

[Farmerford]

Corm:

I had the same questions. The I noticed that my little Kubota B2400 FEL has 1" pins in 2" long bushings for a lift capacity of less than 1000 pounds at the boom ends, while my JD 455D crawler has 1 3/4" pins in 3" long bushings for a lift capacity of over 10,000 pounds at the boom ends. I realize that many factors other than lift capacity determine the load on a pin (torque, alignment of bushings and pin bosses, etc), but in general I use 1" pins in a 1 1/2" to 2" long bushing for small projects (grapple pivots) and 1 1/2" pins in a 2" to 3" bushing for big projects ("A" boom on the rear of a Caterpillar D3B; king pins for mower wheels).

It also occurred to me that the lift pins on a Category II three point hitch are only 1 1/8", and they are usually fastened at only one end. With attachments like turning plows and box blades, they get a lot of abuse from 100hp/8,000# tractors, yet seem to last a long time.

You don't save much material or effort by using pins smaller than 1" and pins over 1 1/2" seem to be clearly overkill for home projects.

The breaking strength of the material is not the critical factor in my cases. It is the wear of the pin and bushing. On smaller projects, rather than go to the trouble of boring (and honing) bosses to take replaceable bushings, I use DOM tubing welded to the arm for the bushing and then phosphor bronze for the pin. The bronze is not very expensive, is about as strong as mild steel (stronger if tempered), and wears before the mild steel bushings. I have used bronze pins in a small pallet loader for several years and they show very little wear. I drill the pins for a grease zerk and use a 5% moly grease. It is certainly not as elegant as the design (and workmanship) others adopt, but it seems to work pretty good for me.

 

[Corm]

Thank you folks for your replies. You've confirmed what I've been thinking, that is 1" pins with about 2" of 1/4" bushings seem to be about right for most projects to be used on our size tractors. I like the tip about using harder steel for the DOM bushings so the pins wear before the bushings do. I've never inquired at my steel supplier about harder steel for the DOM tubing I've bought in the past. I will on my next trip there!

I also never considered bronze for the pins. Never figured it would be strong enough (my lack of experience is showing...). Using them sounds appealing to me, especially coupled with hard DOM tubing. I really like the sounds of replacing the pins and not having to worry about boring out or replacing egg shaped bushings.

Again, thanks for all your input. This probably sounded like a pretty basic question for some of you folks, but I've learned from your responses. Much appreciated!

Corm