slip eccentric offset

[michael burton 1Participant @michaelburton1]

[michael burton 1Participant @michaelburton1]

hi on the model im building it uses slip ecentric valve gear, now the orignal design is a 1.5'' disk with a 1/8'' deep grove so giving 1 1/4'' running surface with a 1/8'' off set bore.

now the stroke and bore have been enlarged as the model is a fair bit bigger so now has a 2 inch stroke how do i work out the new ecentric off set and size

i tried working it out and got no were how does the ecentric size relate to the off set the port spacing is yet undecided but would imagine the ports would be around an inch outside to outside im still new to this so i would like some guidence about this

it may not be a straight forward answer but im all ears for sugetions of how to come up with an answer

i did come up with a disc of 2'' od but this would look stupid on the model so mayby theres a way around this with a diff offset and final diameter

atb mike

[JasonBModerator @jasonb]

I think you would be better off working out the sizes and spacing of your valve ports and then making the eccentric throw to suit.

Out of interest the eccentrics on my 2" Fowler are 1 1/4" dia with 3/16" offset giving 3/8" throw

J

[michael burton 1Participant @michaelburton1]

cheers jason

[nigel jones 5Participant @nigeljones5]

If thats not cleared it up for you, the eccentric has nothing to do with the bore or stroke, it only relates to the spacing of the ports. When you know the port spacing you can design the offset, and from that you can determine the diameter. Be mindful when deciding on the diameter to take into account the space required around the eccentric and ensure there is clearance for the sheath.

[michael burton 1Participant @michaelburton1]

right ok i have some questions so i can try and get my head around all this lets say just for an example that the 2 steam ports were one inch apart and were 1/4'' wide …………… the throw is that measured overall so would be 1'' with a off set to give you a movemant 1/2'' eaither side of centre how would you come to the point of knowing what the off set should be to give you that 1/2'' movemant eaither side that is required and and the relationship to the diameter off said eccentric

i picked these measurments to make my life simpler for understanding ( i hope )

im trying to get my head round this and have searched on the net but came up a blank

i could ask someone to just give me the answer but then i wont learn so if any one could explain using these measurments how to come to an answer it would defo be apprecaited go easy and hopefully itl come to me soon enough

thanks for your time and understanding

[JasonBModerator @jasonb]

The offset of the eccentric gives half the movement, just like the throw of a crank gives half the stroke.

So to get your 1" of movement the centre of the hole will need to be 1/2" offset from the centre of the eccentrics OD.

Overall size is determined by the offset, diameter of the crankshaft and a safe amount of metal to leave between the hole and the narrowest part of the eccentric, a wider eccentric may allow this last dimension to be reduced.

[michael burton 1Participant @michaelburton1]

now i get it brain was working over time and simply over complicating things thank you jason

[nigel jones 5Participant @nigeljones5]

best draw it out by hand on paper first, I have designed them in my head before and then when half machined realised that I hadnt left enough 'meat' between the crank edge and the outer edge of the eccentric, thus when you add grub screw and tighten something will crack!

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