Someone once told me that speed was a determining factor but I tend to disagree as there are examples of both types in slow and high speed service.  One interesting example is the N&W J class which I believe used a single for the first five (600-604) then two thereafter on what was arguably the most powerful 4-8-4 ever.  My best guess would be that is has something to do with reduced friction/improved fuel economy but I honestly don’t know.  Can anyone knowledgeable shed some light on this?

Thanks

Original Post

There's been all sorts of debates of the two "single guide crosshead designs, Laird and Multiple Bearing, versus the double guide "Alligator" design.  As the OP noted N&W used the multiple bearing design on the first five J's (600-604), plus the first ten class A's (1200-1209).  For the rest of those two classes they went to the Alligator design.  I believe that they found the Alligator design to either be easier to adjust, or more rugged (or maybe both).

Meanwhile on the New York Central, the class L-4a Mohawks had Alligator, while the L-4b class has multiple bearing crossheads.

On the Southern Pacific, all of their modern power used multiple bearing crossheads, except the Cab Forwards, because with the reverse running of the drives, the mainrod would exert greater force on the lower crosshead guide of the Alligator crosshead, while using a multiple bearing crosshead would have had more problems because of that same issue.

Stuart

 

The light at the end of the tunnel is the headlight of an on coming train!

Stuart posted:

There's been all sorts of debates of the two "single guide crosshead designs, Laird and Multiple Bearing, versus the double guide "Alligator" design.  As the OP noted N&W used the multiple bearing design on the first five J's (600-604), plus the first ten class A's (1200-1209).  For the rest of those two classes they went to the Alligator design.  I believe that they found the Alligator design to either be easier to adjust, or more rugged (or maybe both).

Meanwhile on the New York Central, the class L-4a Mohawks had Alligator, while the L-4b class has multiple bearing crossheads.

On the Southern Pacific, all of their modern power used multiple bearing crossheads, except the Cab Forwards, because with the reverse running of the drives, the mainrod would exert greater force on the lower crosshead guide of the Alligator crosshead, while using a multiple bearing crosshead would have had more problems because of that same issue.

Stuart

 

Thank you for your response.  Just for clarification, are Laird and MB the same?  I’m guessing the name “Multiple Bearing” refers to the fact that a single support guide must be strong enough to endure thrust from multiple directions.  Is reduced friction even a consideration?

Rich Melvin posted:

It’s just two different ways of accomplishing the same thing. Put the bearings on the top and bottom or put them all on top.

Are we talking roller bearing?  I thought this part of the mechanism was typically a well-lubricated slide.

PRR 5841 posted:
Rich Melvin posted:

It’s just two different ways of accomplishing the same thing. Put the bearings on the top and bottom or put them all on top.

Are we talking roller bearing?  I thought this part of the mechanism was typically a well-lubricated slide.

What Rich means is, the Laird multiple bearing crosshead guide design places those multiple bearing slide surfaces above the piston rod, i.e. "on top". While the Alligator crosshead design has the bearing slide surfaces above AND below the piston rod, i.e. "on top AND bottom". Roller bearings are NOT involved in the crosshead guides.

PRR 5841 posted:
Stuart posted:

There's been all sorts of debates of the two "single guide crosshead designs, Laird and Multiple Bearing, versus the double guide "Alligator" design.  As the OP noted N&W used the multiple bearing design on the first five J's (600-604), plus the first ten class A's (1200-1209).  For the rest of those two classes they went to the Alligator design.  I believe that they found the Alligator design to either be easier to adjust, or more rugged (or maybe both).

Meanwhile on the New York Central, the class L-4a Mohawks had Alligator, while the L-4b class has multiple bearing crossheads.

On the Southern Pacific, all of their modern power used multiple bearing crossheads, except the Cab Forwards, because with the reverse running of the drives, the mainrod would exert greater force on the lower crosshead guide of the Alligator crosshead, while using a multiple bearing crosshead would have had more problems because of that same issue.

Stuart

 

Thank you for your response.  Just for clarification, are Laird and MB the same?  I’m guessing the name “Multiple Bearing” refers to the fact that a single support guide must be strong enough to endure thrust from multiple directions.  Is reduced friction even a consideration?

The Laird and multi bearing crossheads are two different designs.  Santa Fe made the Laird on many of their modern designs, such as the 3751 class 4-8-4 (Santa Fe 3751 Laird crosshead).  As you can see in the picture the Laird crosshead guide has two horizontal bars with a slot for the crosshead to hang out of the side.

The multiple bearing (MB) crosshead was used by many railroads, such as Union Pacific on their modern designs, such as the FEF-3 (844) (Multiple bearing crosshead on UP #844).  On the MB crosshead guide the slot was beneath the guide, with the crosshead hanging below it.

I hope this was helpful.

Stuart

 

 

The light at the end of the tunnel is the headlight of an on coming train!

PRR 5841 posted:

 N&W J class which I believe used a single for the first five (600-604) ....  

You are correct, yet the latest Lionel Legacy versions of both the 600 and 603 are shown in the catalog with Alligator guides. It may not matter to some, but that was enough to keep that $1450 in my wallet.

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