timber grab, help

[drythropple]

I am making myself a timber grab, it has to be the type with a horizontal cylinder to save hight, all the main fabrication is done but I am having trouble with the geometry for the link that goes between each half to make them close/open together, one side is either too fast or too slow and when I get it right I dont have enough movement for them to close right, so much for it being a simple job.
Has anyone got any drawings or web links please, any help or ideas will be most welcome, thanks