At the farm we go through 100's of grease tubes in a years time. With all the equipment we probably have over a thousand zerks to maintain so I'll share some advice with everyone, some has been mentioned above already.
1. Always make sure and clean the zerk with a rag before greasing. Otherwise you can inject dirt into the fitting which can plug the zerk or get dirt into the bearing/joint. The slightest bit of dirt can be quite harmful so this is super important.
2. You have to hit a zerk with a straight alignment. We have some adapters made up with extensions and angle fitting to help with the tricky ones, but most of the time this is unneeded.
3. Sometimes in order to get grease to go in, it is necessary to change the position or take the load off the joint. For example on our tractors with 2wd front axles, it is necessary to jack the front of the tractor up from the frame to take the weight off the axle. We found out the hard way that even though we were greasing these zerks, the grease failed to make it to the top side of the pins which eventually resulted in failure and remachining the parts. If the location the grease enters is "pinched off" you wont be able to get grease to push through hence feeling the zerk is "clogged".
4. Zerks can get plugged. Best way to clean them is to remove them and soak them in penetrating fluid and use compressed air, tip cleaners, etc or simply try a new fitting. Fittings can be threaded or drive in style. NOTE: Never heat a grease zerk with a torch! The ball-check valve inside can and will shoot out like a miniature bullet. People have lost their eyes trying this!
5. Grease gun tips wear out, sometimes it is simply time to replace the tip but for someone at home only greasing autos, I'd think a quality tip should last decades.
6. Grease gun selection and tips: Personally I've never like the "lock-on" style fittings. They are bulky and clumsy when a fitting is in a tight location. I like to use a manual PISTOL style gun that way one hand can pump and the other can keep the tip on the zerk fitting. The manual guns with a lever are a PITA and unhandy. I'm not a fan of electric grease guns either for several reasons. So if you don't have one this is what I'd recommend.
7. Buy an extension hose. None of the grease guns come with long enough hoses. Double your hose length, this makes greasing in tight places much easier.
8. For those who have had issues priming a new tube, this is the best approach: Insert new grease cartridge, thread tube into head of the grease gun only partially. Then ram the rod multiple times. then finish threading the rest of the way in. This method just seems to work.
9. Over greasing can be bad! Over greasing can blow out seals and bearing caps. If this happens, never greasing the fitting would be better so know your limits.
