While a sudden issue is usually easily repaired, a consistent issue that has gone unaddressed for months or years will likely require a total replacement. The problem is that garage doors have a number of heavy, powerful moving parts. If the door is working as it is designed, it can open and close hundreds and hundreds of times without issues. However, if there is even a small issue in the lifting mechanism that repeatedly influences the movement of the door, you will soon find that the damage caused over those hundreds of lifts can’t be fixed.
There are plenty of garage door accessories to make your garage safer. Sears carries replacement safety sensors that will detect anything in the way and keep it from shutting. If you have a small garage or it's full of gardening equipment, a laser parking assistant will help you find the perfect spot every time. You can set it so it hits the perfect spot on your dash as you pull in. You'll also be able to find new tracks to replace worn opener tracks.
That sagging garage door is getting harder and harder to open. Garage doors, especially older, wooden models, are as susceptible to the ravages of time and gravity as the rest of us. If your aging doors are a little saggy, square them up with the tension rods positioned on the back of the door. The rods are placed diagonally from top to bottom corners and can be tightened at a turnbuckle to straighten out the door--do a little at time to allow the door to adjust to the change. If your door isn't already equipped with tension rods, you can buy them at home centers.
The torsion springs on steel rolling doors work the same way as other torsion springs in the garage door industry. When the door is closed, the spring is wound. The spring is secured to a spring anchor bracket on one end and to the shaft on the other. When the steel rolling door opens, the spring in the barrel supports most of the door weight. The remainder of this weight is lifted by the chain hoist or the operator.