Draft gear load calculation

This forum is dedicated to Riding Scale Railroading with propulsion using other than steam (Hydraulics, diesel engines, gas engines, electric motors, hybrid etc.)

Moderator: Harold_V

User avatar
Erskine Tramway
Posts: 390
Joined: Wed Sep 10, 2014 4:13 pm
Location: South Dakota
Contact:

Re: Draft gear load calculation

Post by Erskine Tramway »

Hi Glenn...

The main reason I use those springs is that they don't compress much. For all I know, they don't move at all. They are more of a cushion than anything. My Heywood couplers are fairly height sensitive, they wouldn't handle the movement, between loaded and empty, in the original steel springs that came with the castings. Sir Arthur's full sized cars rode on a flat piece of rubber between the journal box and pedestal. I never considered the load rating, I just picked springs that fit the locating recesses. Right this minute, I can't even remember where I bought them, let alone anything else about them...and I need to get some more for the tool car I'm planning :shock:

I have longer ones on my passenger car, so they might compress a little more, though the loads on the car are not as much as I put on one of the freight cars. I can't say how the passenger ride is, I've never ridden in the car, but nobody's complained...the seats are fairly soft.

Our drawbar arrangement at Sandley's was a bolt screwed into the rear of the coupler shank. The bolt went through a hole on a cross member in the frame, with a steel coil spring on either side of the cross piece. It serves the purpose.

Mike
Former Locomotive Engineer and Designer, Sandley Light Railway Equipment Works, Inc. and Riverside & Great Northern Railway 1962-77
BN RR Locomotive Engineer 1977-2014, Retired
rkcarguy
Posts: 1730
Joined: Tue Aug 22, 2017 10:33 am
Location: Wa State

Re: Draft gear load calculation

Post by rkcarguy »

Erskine Tramway wrote: Wed Sep 29, 2021 10:45 am Hi Glenn...

The main reason I use those springs is that they don't compress much. For all I know, they don't move at all. They are more of a cushion than anything. My Heywood couplers are fairly height sensitive, they wouldn't handle the movement, between loaded and empty, in the original steel springs that came with the castings. Sir Arthur's full sized cars rode on a flat piece of rubber between the journal box and pedestal. I never considered the load rating, I just picked springs that fit the locating recesses. Right this minute, I can't even remember where I bought them, let alone anything else about them...and I need to get some more for the tool car I'm planning :shock:

I have longer ones on my passenger car, so they might compress a little more, though the loads on the car are not as much as I put on one of the freight cars. I can't say how the passenger ride is, I've never ridden in the car, but nobody's complained...the seats are fairly soft.

Our drawbar arrangement at Sandley's was a bolt screwed into the rear of the coupler shank. The bolt went through a hole on a cross member in the frame, with a steel coil spring on either side of the cross piece. It serves the purpose.

Mike
I think Glenn tried this and had trouble with the bolts breaking off. It works though if there is adequate clearance inside the coupler pocket so the bolt doesn't bind as the coupler swings side to side.
I believe somewhere in your thread you said those rubber springs were a McMaster-Carr item?
User avatar
Erskine Tramway
Posts: 390
Joined: Wed Sep 10, 2014 4:13 pm
Location: South Dakota
Contact:

Re: Draft gear load calculation

Post by Erskine Tramway »

rkcarguy wrote: Thu Sep 30, 2021 1:39 pm

I think Glenn tried this and had trouble with the bolts breaking off. It works though if there is adequate clearance inside the coupler pocket so the bolt doesn't bind as the coupler swings side to side.
I believe somewhere in your thread you said those rubber springs were a McMaster-Carr item?
I thought I got them at McMaster, but I can't find them in their catalog now. I'll have to hunt around and find the bill. Anyway, our bolts at Sandley's were about 3/4" diameter, and the hole in the cross piece was considerably larger. The coupler was centered by the compressed spring pressure on the mounting plate. The coupler rested on the bottom of the opening in the end beam casting, and there was plenty of swing available. Of course, and I suppose this is fairly critical, we were running on a standard gauge alignment. We didn't have any sharp curves, except through our #10 turnouts, so we didn't need a lot of swing.

Mike
Former Locomotive Engineer and Designer, Sandley Light Railway Equipment Works, Inc. and Riverside & Great Northern Railway 1962-77
BN RR Locomotive Engineer 1977-2014, Retired
Glenn Brooks
Posts: 2930
Joined: Mon Nov 10, 2014 1:39 pm
Location: Woodinville, Washington

Re: Draft gear load calculati

Post by Glenn Brooks »

Thanks for the info Mike. I’ve found several suppliers for similar solid, round polymer “springs” . some are solid, some have a center hole formed through the middle. Usually all have factory ratings for deflection and max load bearing capacity- so this seems to answer the important question of “will these carry the proper amount of weight? Iam still struggling to find “cushioning ability” for passengers. I think I will just buy an assortment of diameters, lengths, and load bearing capacity and experiment... the length of the spring compression seems to be the key for softening shock . If I understand this correctly, the ARR standardizes class 1 Coupler systems at 1” initial slack in the draft gear and something like 4” of compression under full load.

Also, Yes, as Ryan mentioned, I did have an almost immediate failure of the threaded yoke I first used to mount draft gear on new to me 2.5” scale couplers. I initially set the couplers up with a yoke, consisting of 3/8” (grade 3) hardware-store-variety threaded rod.

One of the yokes broke within the first 10 minutes of operation. The breakage was right at the location where the threaded rod pivoted around the hole in the anchor sill. So a stress break at point of contact between the coupler gear and the fixed draft gear sill attached to the frame.

I removed all the 3/8” coupler rods and replaced with grade 8, 1/2” smooth shanked bolts (no threaded risers near the springs or mount point). No problems since then.

Glenn
Moderator - Grand Scale Forum

Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....
rkcarguy
Posts: 1730
Joined: Tue Aug 22, 2017 10:33 am
Location: Wa State

Re: Draft gear load calculation

Post by rkcarguy »

Erskine Tramway wrote: Fri Oct 01, 2021 10:28 am
rkcarguy wrote: Thu Sep 30, 2021 1:39 pm
I thought I got them at McMaster, but I can't find them in their catalog now. I'll have to hunt around and find the bill. Anyway, our bolts at Sandley's were about 3/4" diameter, and the hole in the cross piece was considerably larger. The coupler was centered by the compressed spring pressure on the mounting plate. The coupler rested on the bottom of the opening in the end beam casting, and there was plenty of swing available. Of course, and I suppose this is fairly critical, we were running on a standard gauge alignment. We didn't have any sharp curves, except through our #10 turnouts, so we didn't need a lot of swing.

Mike
I was looking for these elastomeric springs on McMaster also and had no luck as well. Maybe they have a different name or have gone the way of no longer available, as many things have since Covid?
I think the elastomeric round stock with the hole in the middle that they offer has potential, it could have a taper ground on one end to make it "progressive" in stiffness. I'll upload a sketch when I get a free moment.
Glenn Brooks
Posts: 2930
Joined: Mon Nov 10, 2014 1:39 pm
Location: Woodinville, Washington

Re: Draft gear load calculation

Post by Glenn Brooks »

Ryan, as a side note, most (maybe all) polymer springs, including the “elastomeric” type , that I have read about are “progressive” because of the type of material they are composed. Apparently compression produces heat, which then softens the body of the spring, giving the spring greater elasticity- which absorbs progressively greater shock loading as compression forces increase. Apparently this occurs logarithmically, rather than linearly. Hence polymer springs are said to produce 50% greater buffering than comparable steel springs.

So the greater the compression force, the higher the buffering action. Now, if this true for the elastomeric type materials, you would not need to alter the manufactured shape of the spring (machining a cone in the center hole, for example), as machining a cone shaped ID would reduce the capacity of the spring in an amount equal to the quantity you remove. In these type of ID center holes, during compression, the center void closes, as the spring body bulges both inward into the center void, and outward from the OD.

I have seen that each category of polymer spring (length, diameter, durometer rating e.g. material hardness) has a distinct load rating. So an alternative to machining some part of the spring, is simply buy a different material hardness at your desired height - to achieve the compression that you require...

I’d be very interested to learn whatever you might discover if you dig deeper into these types of materials.

Glenn
Moderator - Grand Scale Forum

Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....
rkcarguy
Posts: 1730
Joined: Tue Aug 22, 2017 10:33 am
Location: Wa State

Re: Draft gear load calculation

Post by rkcarguy »

Glenn, I was basing my taper on looking at automotive bump stops. They are almost always cone or wedge shaped. But I have noticed when they wear, it's always the narrow portion that is squished and/or deformed. I think you are correct, that a cylinder of solid material of the right softness would squish over its entire length, still be a "progressive rate", and probably last longer.
My thought for my ballast car, was to use them inside the springs, and cut them maybe 1/4" shorter than the springs when compressed into the side frame openings. When empty, the car would then have some travel on the springs and a smooth ride, and would then rest on the rubber "stops" when loaded with a scoop of gravel and still have some give without sacking the suspension so much that coupler height became an issue.
I'd be interested in the amount of squish noted for weight applied, it doesn't look like they really offer any of that information in the specs. It would be easy enough to make a simple test fixture and stack lifting weights on a chunk of the stuff and see how much weight results how much deflection. I think about 1/4" compression for 400-500#'s (x8) would be ideal for a ballast car for a 3200-4000# gross weight, and cut that in half for passenger cars.
Glenn Brooks
Posts: 2930
Joined: Mon Nov 10, 2014 1:39 pm
Location: Woodinville, Washington

Re: Draft gear load calculation

Post by Glenn Brooks »

Ryan,

Here are a couple of polymer spring suppliers that may have something similar to what you are thinking about:

https://pleiger.com/high-density-flexib ... hane-foam/

https://www.acrotechinc.com/tubes-springs/

There are other polymer suppliers, with charts listing individual size, loading weights, compression rates and durometer (hardness) ratings. But somehow I didn’t bookmark them correctly...and didn’t save their URL.

Glenn
Moderator - Grand Scale Forum

Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....
Post Reply