Steerable truck variants

[bogieman]

One thing to remember is that EMD had to go with a steerable truck because of their one inverter per truck design. Since with one inverter per truck all three traction motors will turn at the same RPM, EMD had to use the steerable truck in order to keep all the wheels as close as possible to the same diameter or the differences in torque would cause problems. Since GE went with one inverter per motor, the need for the steerable truck wasn't as critical for them.

Stuart

This isn't really correct. When EMD introduced the 70 series models, first DC, then AC traction, they were only offered with the HTCR radial truck and no interchangeable non-radial designed existed. The HT-C truck was not an option as it could not fit the longer distance of the D90 and AC traction motors to the nose support. With the introduction of the SD70M-2 and SD70ACe in 2005, the HTCR-4 radial (which mostly differs from the HTCR in the means interlocking the truck to the underframe) became an option and the HTSC-2 became standard to offer a lower cost competitor to GE who was selling most of their locos with their non-steering truck. The one inverter per truck system works fine with a non-steering truck but most of EMD's customers opted to pay more and get the advantages of the radial truck - less wheel flange and rail wear and increased adhesion in curves. UP and NS ordered all of their SD70ACe's with radial trucks while BNSF initially chose the non-steering option but later switched their EMD orders to the radial truck.

Dave

[es80ac]

Dave, out of idle curiosity, do you know what kind of difference there is on the truck center pins used by the contemporary GE and EMD trucks? I have seen photos each and I have seen first hand the GE trucks on several occasions while I was at the Erie plant. GE seems to use a long pointed center pin mounted on the frames which will then mate with the truck. The EMD pin seems like a shorter but larger diameter cylinder that goes into the center of the truck. Do you happen to know the maximum draft force each of these can withstand?

Also, why does European trucks tend to favor traction bars type of mechanism to transfer the force between the trucks and the frame? This type of design do not use center pins. Thanks

[bogieman]

The pins are quite similar and extend in to a receiver on the truck. The GE pin is smaller in diameter where it engages the truck but they are at a similar height above rail. Both builders offer 200,000 lbs. starting TE so the pins must be designed to handle more than 1/2 of that as fatigue load to account for weight shift and it's affect on TE split between trucks. Their ultimate strength in a derailment will be much higher than that.

On the GE truck, the pin receiver is a block within a roughly square cut-out in the first transom of the truck frame with a pre-loaded rubber compression pad on front and rear of the block, which allows for lateral and yaw motion of the truck frame relative to the loco frame. On the EMD truck, there are a pair off links connecting the pin receiver to the first transom so there is similar lateral and yaw freedom but the pin sees little lateral force on the EMD design.

If you are referring to European designs using outboard traction connections between the truck and loco frame, it is at least partly due to the carbodies having more strength in their sidewalls from monocoque construction compared to the heavy bottom plates on the frames of the NA locomotives.

Dave

[es80ac]

Thanks Dave, very informative and helpful.

[Engineer Spike]

The GE units, with steerable trucks seem to sway along, while going down the track. I don't know it it is the design, or the mechanical department isn't keeping the trucks properly serviced. They could have warn out bushings, or the front and rear wheel sets are rotating about the center set, due to any slight misalignment of the track.

CPR got rollerblades on the first order of AC4400, and continued through the first order of ES44. There was a bulletin which accompanied the second order, and explained the differences, such as returning to conventional control stands. I believe it mentioned the maintenance broblems with the steerable trucks, and the return to rollerblades. One strange thing was that I had a first order ES (low 8700 class). It had been retrucked with blades.

[MEC407]

I've seen several videos of SD90MACs (both the 710-powered version and the 265H-powered version) which showed them hunting quite violently when operating at typical big-railroad track speeds. Was there something unique to the SD90MAC steerable trucks that made them hunt so badly? I haven't seen that when I've watched videos of GEs with steerable trucks or when I've observed those GEs in person.

[bogieman]

The SD90MAC's have an unfortunate coincidence of underframe first bending frequency, truck hunting, and cab resonance on the isolators that made for a rough ride on the UP. The bigger, heavy traction motors in the HTCR-2 truck makes it more susceptible to truck hunting and that was further made worse by the UP practice of building their track to the low side of the allowable gauge, which lowers the hunting threshold - the mainline from North Platte to Omaha was the worst. There was a modification to tighten the lateral axle to truck frame clearance at the end axles that raised the hunting speed significantly. SD70ACe's have a smaller, lighter traction motor and a stiffer underframe so the frequencies don't coincide. When the isolated cab was re-introduced on the SD70ACe, the isolator arrangement was changed compared to earlier isolated cabs, which also keeps the resonant frequencies apart.

The radial trucks do their job in extending wheel life by reducing flange wear so the condemning wheel condition is changed from thin flange to high flange generally. The longer wheel life leads to more hollowing of the tread which increases the effective conicity of the wheel to rail which does lower the speed for truck hunting.

Dave

[MEC407]

Thanks, Dave!

[es80ac]

The SD70ACE frame is noticeably beefier than the SD90 frame， if we looks at the bottom plate (i forgot the name of it), it is almost twice the thickness of the SD90 frame bottom plate. That must been in direct response to SD90 frame flexing issue?

[bogieman]

The bottom plate thickness is part of it but the underframe height is the most critical factor. On the original SD80/90MAC's the underframe depth at the cab end was limited by the height of the cab in order to stay within the clearance design requirements - aft of the cab, the underframe steps up with a continuous bottom plate. The bigger 45" wheels on those locos raised the bottom plate further limiting the underframe height. The SD70ACe had a new cab design, as well as 43" wheels, that allowed the underframe to have to a constant section height greater than the SD80/90MAC which significantly raised the first order bending frequency.

Dave

[es80ac]

The bottom plate thickness is part of it but the underframe height is the most critical factor. On the original SD80/90MAC's the underframe depth at the cab end was limited by the height of the cab in order to stay within the clearance design requirements - aft of the cab, the underframe steps up with a continuous bottom plate. The bigger 45" wheels on those locos raised the bottom plate further limiting the underframe height. The SD70ACe had a new cab design, as well as 43" wheels, that allowed the underframe to have to a constant section height greater than the SD80/90MAC which significantly raised the first order bending frequency.

Dave

Dave, are you referring to the first batch of SD90MAC? The second batch SD90MAC-H has the new cab design and appears to sit higher, and I suppose the underframe is different?

[bogieman]

That's correct. The later 90MAC Phase II underframe was completely different having an integral fuel tank.

Dave

[es80ac]

That's correct. The later 90MAC Phase II underframe was completely different having an integral fuel tank.

Dave

thanks Dave! Is the integral fuel tank meant to strengthen the frame? Did the phase II have any frame flex as far as you know?

Also I am curious do you have any input on the strength of welded trucks vs cast trucks? Which is stronger and better?

[Engineer Spike]

I’ll go on record to say that truck design is not one of GE’s strong suits. The steerable trucks seem to make the units sway side to side. This isn’t a problem on the EMD radial truck. I have however seen a stencil on a UP about having the steering mechanism pinned. Maybe this way a way to save repair costs on the EMD truck. My point was that given the GE ride, it doesn’t surprise me that the truck components wear out quickly.

Don’t get me started on the FB trucks.

[bogieman]

There seems to be some confusion as to what modification to the HTCR trucks was done. In my original design (patent 4,765,250), there is a link (known as the interaxle link, item 51 in the patent drawing) connecting the steering beams at the first and third axles to couple them to rotate in opposite directions. When the 1,000 unit SD70M order from UP was in the engineering phase, there was a big effort to find cost savings. We knew from the HTCR development testing in the late 80's that the force in the interaxle link was lower than originally forecast. At each end of the link there was a large (2" ID) spherical bearing that was oversized for the application. As a cost savings, the spherical bearings were downsized to 1.25" ID which proved to be to much of a load capacity reduction and those bearings wore significantly in about 5-7 years of service to render the link ineffective in providing any coupling of the axle motions. Since the worn bearings weren't detected until the locomotives had significant mileage and the trucks were not observed to have any operational or wheel wear issues, a test program where the link was totally removed from several locomotives was conducted for over a year. As no difference in performance or wheel wear was observed during the test period, the EMD recommendation was to remove the interaxle link at overhaul. That is what the UP stencilling denotes. A production change was made about the same time (2008) on new locomotives to remove the link and all HTCR trucks produced since then do not have the interaxle link. When I designed the fabricated frame HTCR-6 truck for the Tier 4 locomotives (see patent 9,434,395), not having to incorporate an interaxle link made the frame and steering beams much simpler. In summary, these are still radial steering trucks.

Back on topic of GE steerable trucks, GE's design to get stable running was to link the traction motors together in the lateral direction with structures that connect the adjacent nose links and roller support bearing housings at axles 1-2 and 2-3 so the axles would move laterally in unison (patent 5,746,135). Bellcranks linked to the bearing adapters at axles 1 and 3 allow for axle rotation and a very visible linkage on the outside of the truck frame connects the bellcranks to rotate in opposite directions and also is linked to lateral motion of the middle axle. This design does not use axle yaw dampers but relies on the constrained motions laterally and in yaw of the axles to avoid hunting instability. Wear at the connections of these linkages will lower the hunting threshold speed and may be what you experience but I'm not involved with those to be sure. I do know that speaking to shop folks at the CSX Huntington WV shop about 15 years ago that fairly early in their service life the steerable trucks required major work to replace worn out bushings. I don't what upgrades or improvements they may have made. The shop people also complained of the extra work involved with combo replacement on the steerable truck due to the structure connecting to the support bearing housing. These may be some of the reasons CSX stopped ordering steerable trucks on their latest orders.

The EMD HTCR truck family gets stable running by using axle yaw dampers the connect the ends of the steering beams to the truck frame to damp axle yaw motion along with truck yaw dampers between the truck frame and underframe. These yaw dampers have proven to be very reliable and are typically rebuilt at truck overhaul as they are not of welded construction but can be taken apart and rebuilt. Early production HTCR trucks did have a problem with a rubber pad between the axle bearing housing and truck frame that limits lateral axle motion; a redesigned pad solved that problem 20+ years ago.

Dave