coupling rod play

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proto87stores

coupling rod play

Postby proto87stores » Thu Sep 16, 2021 10:56 pm

Since I'm likely to be assembling coupling rods to chassis in a few months time, I'm wondering what the correct side play allowance on the rods on the wheel crank pins should be? Given that horn blocks on adjacent wheels can be alternatively up and down, there must be enough play to accommodate that on top of any running clearance.

My LNER N7 model distance between axle centres for the longer coupling rod is 35.5 mm. If there is a designed in horn block suspension movement of +/- 0.25 mm, then the wheel centres can move an extra 0.0035 mm apart.If the suspension is designed for +/- 0.5 mm movement, then the inter axle distance can increase by 0.0113 mm.

In the latter case would an oval hole in the coupling rod be a practical solution?

Andy

davebradwell
Posts: 1173
Joined: Fri Jan 18, 2019 3:48 pm

Re: coupling rod play

Postby davebradwell » Fri Sep 17, 2021 8:21 am

I can assure you that the 0.01mm change in wheelbase is completely negligible when compared with the clearancw required to accommodate the errors you build in. Start with Difference in centres between coupling rods and axles, crank throw differences, quartering errors, eccentric crankpin bushes, leaning crankpins......and on.

DaveB

petermeyer
Posts: 307
Joined: Tue Oct 01, 2013 10:06 am

Re: coupling rod play

Postby petermeyer » Fri Sep 17, 2021 11:10 am

So how much play would be ideal to allow for all that? 0.1mm? Peter

Daddyman
Posts: 740
Joined: Sat Jun 03, 2017 1:09 pm

Re: coupling rod play

Postby Daddyman » Fri Sep 17, 2021 11:25 am

I always try for 0.05 (with holes in the coupling rod definitely not oval!). So assuming 1.5mm crank pin, the rods need opening with a 1.55mm drill. If that amount of clearance doesn't work, there's something wrong in one of the places Dave mentions.


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