An autofocus system (or AF system) uses a sensor, a control system and a micro-motor to focus. The focus-motor may either be in the lens or in the camera body itself. The ultrasonic motor is an electronic autofocus micro-motor used inside lenses and enables them to autofocus. The screw drive autofocus is opposite from ultrasonic motors where the lens is driven by an autofocus motor in the camera body instead of using an micro-motor inside the lens. The link between the body and the lens is very like the link between a screwdriver and a screw. When the focus motor inside the camera is doing the work it is called screw drive autofocus.
Most of the older AF lenses from Nikkors (Nikon), Minolta, Sony and Pentax use the mechanically screw drive autofocus system. Newer lenses tend to have their own autofocus motors and are driven by the body electrically rather than mechanically. If the micro-motor in the lens have stopped working you have two options; replace it (could be pricy of it is not covered by warranty) or to deactivate the micro-motor and let the focus motor in the camera body do the work.
The technology has been applied to photographic lenses by a variety of companies under different names:
Active AF systems measure distance to the subject independently of the optical system, and subsequently adjust the optical system for correct focus while passive AF systems determine correct focus by performing passive analysis of the image that is entering the optical system. The assist light (also known as AF illuminator) "activates" passive autofocus systems in low-light and low-contrast situations in some cameras. The lamp projects visible or IR light onto the subject, which the camera's autofocus system uses to achieve focus. Many cameras that do not have a dedicated autofocus assist lamp instead use their built-in flash, illuminating the subject with stroboscopic bursts of light. In a hybrid autofocus system, focus is achieved by combining two or more methods, such as active and passive methods or phase detection and contrast measurement.
The ultrasonic motor is used to focus on newer cameras from Pentax while screw drive autofocus is commonly used on older cameras. Some lenses from Pentax have the ability to autofocus with the ultrasonic motor or with the motor inside the camera body. Pentax did this because they wanted autofocus to work on older camera bodys, such as K1000D Super. If the ultrasonic motor in the lens stops working the autofocus stops working on newer cameras but the screw drive focus continues to work on older camera bodys. If the ultrasonic motor stops working you can in other words disable it and let the camera body do the work. When the ultrasonic engine is disabled the lens tells the camera body that it does not have a build-in ultrasonic motor and that the screw drive autofocus in the camera body have to be used instead.
I am going to tell you how you can disable the ultrasonic motor on lenses from Pentax (I have two camera houses from Pentax and did this with my K20D) but I imagine the process is similar for other brands. When disabling the ultrasonic motor on Pentax-lenses you can use either Pentax K-7, Pentax K20D, Pentax K10D, Pentax K-m, Pentax K-x or K100D Super. You should at least have a empty memory card before you start. The battery should be fully charged as well. Here is a list of lenses from Pentax in production as of May 2013 and all with SDM in their names have ultrasonic motor.
Note: These are unofficial instructions and something you do at your own risk.