I have a summit 4500. the belt is off center to the left side and I have use all the adjustment and the belt doesn't move

Good day.
The belt can go off track for a number of reasons. If you say you've used all the adjustment and the belt doesn't move. To me this could mean that the belt either doesn't rotate or it is not moving to the center (centre), or to the right. I presume you mean it doesn't return to the center. But since you have said you've used all the adjustments, that also means that you have actually removed the plastic guards off to have done the supposed adjustments needed to make the belt track back in the center. So, here is the confusing part. Depending on which plastic guards you removed to do the supposed adjustments, normally the belt tracking bolts are at the foot of the treadmill. Stand on the treadmill, looking at the Instrument panel, the foot of the treadmill is behind you. There are two hex type adjusting bolts, one on each side. Now if the belt pulls left, you don't tension the left adjusting bolt, which would stretch the belt and make the treadmill labour worse. A belt that pulls sideways now but not when it was new, shows that the treadmill belt had been neglected. Instead, try slackening the right adjusting bolt to loosen the tension of the belt. An over tightened belt makes the motor work harder, leading to overheating and cook out, plus it also stretches the belt unnecessarily. A lose belt also means it slips at the revolving rotor (motor side) each time you impact the belt with your feet as you run or jog, therefore it means that the slippage will cause the rotor or the belt to wear out. To check on the correct tightness, lift one side of the belt at the center of the platform. It should raise no more that 2 and half inches. if it is less, it is too tight, if it is more, it is too slack. Either way, it is for you to experiments because you could be a six foot five and weight a tonne or you could be a four foot two and weight like a butterfly. Each person is different. Most treadmills are adjusted for up to five foot ten and for those that are taller or bigger, there are treadmills designed for them too. As you know, there are treadmills for all walks of life, small ones, medium ones and large ones, all designed for the appropriate weight ratio in mind. You can't use a small treadmill if you're six foot two and weight 14 stones (90 kg). There are treadmills to accommodate larger people. Most belt failures are the result of... like I said above, a number of reasons. One of them is the failure to maintain a treadmill. Lift the belt and place the palm of your hand on the metal platform (where the belt glides on) and feel if it is sticky, hard to slide the palm of your hand, or feels dry. If any of thei si evident, then your treadmill od labouring and that means the result of the belt running to one side is because it has been stretched as the motor laboriously and continuously been trying to keep going. I have a Stream treadmill and every month of continuous usage, I lubricate the underside of the belt with a quality silicone lubricant, in spray form. I never spray it directly underside, especially if I have carpeted floor, but if you have lino or polished floorboards, I suggest you don't spray it at all because it stains carpets and makes lino and floorboards slippery. Instead, use a lint free cloth. Go outside, spray the lubricant on the cloth to just soak one side, then shape the cloth into a ball, return inside and lift the belt at the center and apply the wet site of the cloth on the platform, not under the belt. Spread the lubricant evenly across the center of the platform but not towards the rollers, just the center. Then run the treadmill a little. Stop it and reapply it again. Repeat the process a third time, then run it again. Then stop it and lift the belt. With the palm of your hand, see if you can feel any moisture or a slippery feel on the platform. If it feels slippery, then the belt is lubricated properly. Do this only every month of daily use. The lubricating of the platform is almost very little. never over lubricate or you'll soon see that excess lubrication stains the machine. You can hear it when the belt runs dry. A belt that is adjusted and lubricated properly runs almost silent, but a dried out belt sounds raspy.