Fast rotary drive for a handheld micrometer
A micrometer drive configuration for a handheld micrometer comprises: a frame; a spindle; a linear displacement sensor that senses a displacement of the spindle; a threaded spindle drive having a relatively coarse thread pitch, wherein the threaded spindle drive is attached to a spindle drive gear; a planetary drive gear that mates to the spindle drive gear; a ring gear surrounding and mating to the planetary drive gear; and a thimble generally surrounding the spindle drive gear, the planetary drive gear, and the ring gear. The thimble is coupled to drive the spindle drive gear through the planetary drive gear, and the planetary drive gear is sized and mounted such that the spindle drive gear turns faster than the thimble.
1. A micrometer drive configuration for a handheld micrometer, comprising:
a frame;
a spindle;
a linear displacement sensor that senses a displacement of the spindle;
a threaded spindle drive, wherein the threaded spindle drive is attached to a spindle drive gear;
a planetary drive gear that mates to the spindle drive gear;
a ring gear surrounding and mating to the planetary drive gear; and
a thimble generally surrounding the spindle drive gear, the planetary drive gear, and the ring gear,
wherein:
the thimble is coupled to drive the spindle drive gear through the planetary drive gear, and
the planetary drive gear is sized and mounted such that the spindle drive gear turns faster than the thimble.
2. The micrometer drive configuration of claim 1 , wherein:
the ring gear is coupled to the thimble and rotates at the rate of the thimble; and
the planetary drive gear is fixed to the frame at a rotary axis of the planetary drive gear.
3. The micrometer drive configuration of claim 2 , wherein the thimble and the spindle drive gear operate at a 5:1 gear ratio.
4. The micrometer drive configuration of claim 1 , wherein:
the ring gear is fixed to the frame; and
a rotary axis of the planetary drive gear is coupled to the thimble and rotates at the rate of the thimble.
5. The micrometer drive configuration of claim 4 , wherein the thimble and the spindle drive gear operate at a 6:1 gear ratio.
6. The micrometer drive configuration of claim 1 , wherein:
the spindle drive comprises threads with a lead angle λ and a thread angle α n measured relative to the normal;
the spindle drive threads and an engaging member that drives the spindle exhibit an effective coefficient of static friction μ; and
the lead angle λ, the thread angle α n , and the effective coefficient of static friction μ satisfy the relation: μ>cos α n tan λ.
7. The micrometer drive configuration of claim 1 , wherein the spindle drive includes external threads that drive the spindle.
8. The micrometer drive configuration of claim 1 , wherein the spindle drive includes internal threads that drive the spindle.