Steerable truck variants

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Re: Steerable truck variants

Postby bogieman » Sun May 01, 2016 6:41 pm

mp15ac wrote: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.

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Re: Steerable truck variants

Postby es80ac » Wed May 04, 2016 10:56 pm

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
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Re: Steerable truck variants

Postby bogieman » Thu May 05, 2016 11:38 pm

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.

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Re: Steerable truck variants

Postby es80ac » Tue May 10, 2016 11:54 pm

Thanks Dave, very informative and helpful.
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Re: Steerable truck variants

Postby Engineer Spike » Tue Jun 27, 2017 3:17 pm

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.
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Re: Steerable truck variants

Postby MEC407 » Wed Jun 28, 2017 8:16 am

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.
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Re: Steerable truck variants

Postby bogieman » Wed Jun 28, 2017 9:01 pm

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.

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Re: Steerable truck variants

Postby MEC407 » Thu Jun 29, 2017 12:15 pm

Thanks, Dave!
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Re: Steerable truck variants

Postby es80ac » Sat Jul 01, 2017 4:34 pm

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?
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Re: Steerable truck variants

Postby bogieman » Sun Jul 02, 2017 12:48 pm

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.

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Re: Steerable truck variants

Postby es80ac » Mon Sep 04, 2017 11:12 pm

bogieman wrote: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?
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Re: Steerable truck variants

Postby bogieman » Tue Sep 05, 2017 8:32 am

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

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Re: Steerable truck variants

Postby es80ac » Fri Sep 08, 2017 12:55 am

bogieman wrote: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?
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