I have a Rancher 55 18" with a new chain. Started

You may have a bent bar, or you could be putting the chain on a little too loose. Perhaps the chain is not getting lube ?
Possible that the chain tentssioner and or adjustment is loosening up as you are using the unit. Other than that maybe a defective new blade ?

indicates incorrect length chain in use
the adjustment slots should be about 1/3 when fitted moving close to 1/2 when adjusted
the correct adjustment is when the chain in the center of the bar is lifted up the bottom segments of the chain are still in the bar groove
make sure that the oiler hole is not blocked from saw dust and chips

Did you try and pull back the chain lock shield. its the handle abouve the bar, if you hit it it will lock the chain. Pull it back

If you have the original chain, either measure it or count the links to be certain that the new chains match as they should, or just buy a chain a few links shorter.

Not enough oil getting to the chain, check the oil flow, hold the tip of the bar close to a tree stump rev the saw, you should see a good line of oil on the stump srayed from the end of the bar, if not the machine is not oiling, drain the oil tank, fill with a suitable cleaning fluid, give it a good rinse out, and try again.

It sounds like your chain is just dull.Or might not be sharpened right.This usually happens when the chain is getting dull.

The normal reason for a saw to cut to the left, or right is because the cutters have been sharpened more on side than the other, they are now longer on one side, this then sends the cut off to that side. Measure the cutter lenghts, take the correct file and shorten the longer cutters

The chain may have the procedure but the driver plate is worn out - what happens when you are not attached to the chain lubrication oil lubrication channels or if the chain is about or when the pump is not working.

If you look under the machine by the chain you'll see a flat head screw (next to the sprocket) which controls the bar and chain oil flow . Try to turn the screw and check if there's any oil flow to the chain . If not you will need to replace the oil pump (part # 537 10 55-02 which will cost you about $35.00) Hope this help

It depends on the chain you have installed on the chainsaw. You can look at the numbers stamped on the drive link side of the cutting chain and it should have a 2 digit number that correlates to the size.

The factory chain supplied with the #455 rancher was a .325" pitch 3/8" chain.

Here's a link to the specs for your saw with some useful info

http://www.husqvarna.com/us/homeowner/products/sawing/chainsaws/455-rancher/#specifications

Hope you found this very helpful and best regards!