My understanding is that packing is due to the steel balls getting jamming in between the lifter bars, and this could probably happen at the shell and ends. The remedy is to design the lifters so the space in between bars isn't an even multiple of the balls diameter. I recall there being a good paper from the SAG 2006 conference.

You know packing usually happens in WET SAG mills and as you mentioned one of the solutions is increasing space between lifters but I was wondering that does it happen in DRY SAG mills too? We are trying to change head radial lifters in a dry SAG mill.

I don't have experience in dry SAG mills, but would expect that the function is the same whether the mill is operated wet or dry.

There are two types of "packing".

One is, as suggested above, the jamming of steel balls in between liners, where geometry of the liners facilitates.
The other is packing of wet ore in between the liners, which cause the "lifting" function of the liners to be severely compromised. I am not too sure what is causing this to happen, I am more inclined to believe is the nature of the ore and % of water addition. The shape and size of the pocket between the shell lifters will have an influence, not sure how much though.

If you have SAG "dry" application, where % of moisture in the ore is very, very low and the charge is fluidized with ventilated air, I would say the chance of packing is very small.

Then again, packing of shell liners will have major effect on the charge motion, while packing of F/E liners, shouldn't compromise grinding too much other than reducing available volume.

In my view, it should not matter whether the mill is dry or wet. Packing will result if the gaps between the liners permit it, and if the particle size produced in the mill will fit in these gaps. We see packing in our dry vertical roller mills and also in cement mills. Granted the packing is not as severe as in wet mills, as the presence of water can cause some ores to behave like a self-setting concrete mix. Liner design in terms of lifter spacing should consider the best spacing for charge lift. Quality of liner fit up and casting accuracy will determine the spacing between liners. Proper design and use of rubber wedges will prevent packing to some degree, but packing is not necessarily a bad thing, think of it like an autogenous wear layer.

In dry SAG mills the head lifters play important role. The ground materials should be distributed on suction area. Part of this job should be done by head lifters.