For lifting systems and materials
handling situations, shaft-based power often needs to be converted to
linear motion. This may involve screws, winches, hoists and associated
IntroductionElectric motors provide rotary motion (shaft power). Pneumatics and hydraulics usually give a linear motion. To use an electric motor to produce linear motion, a power screw is usually used. Other alternatives include chain drives, timing belts, and rack and pinion gears.)
An electric linear actuator. Rotary motion from the motor drives a screw that produces linear motion at the output.
|Advanced Notes: Power Screws: (Chapter 15 Mech Design Data Book). power_screws.pdf|
A typical power screw
Power Screws are used for providing
motion in a smooth uniform manner. They are linear actuators that
transform rotary motion into linear motion. Power
screws are are generally based on Acme, Square, and Buttress
threads. Ball screws are a type of power
screw. Efficiencies of between 30% and 70% are obtained with
conventional power screws. Ball screws have
efficiencies of above 90%.
Power Screws are used for the following three reasons
- To obtain high mechanical advantage in order to move large loads with minimum effort. e.g Screw Jack.
- To generate large forces e.g A clamp or vice.
- To obtain precise axial movements e.g. A machine tool lead screw.
This form is used for power/force
transmission i.e. linear jacks, clamps.The friction
is low and there is no radial forces imposed on the mating
nuts. The square thread is the most efficient
conventional power screw form. It is the most
difficult form to machine. A split nut cannot be engaged/disengaged on a square thread (unless it was very worn).
Acme or Trapezoidal form
Used for power transmission i.e. lathe lead screws. Is easier to manufacture compared to a square thread. It has superior root strength characteristics compared to a square thread. The acme screw thread has been developed for machine tool drives. They are easy to machine and can be used with split nuts. The thread has an optimum efficiency of about 70% for helix angles between 25o and 65o. Outside this range the efficiency falls away.
A strong low friction thread. However it is designed only to take large loads in on direction. For a given size this is the strongest of the thread forms. When taking heavy loads on the near vertical thread face this thread is almost as efficient as a square thread form.
Recirculating Ball ScrewThis type of power screw is used for high speed high efficiency applications, and is becoming more common. It has become popular because it is essential in CNC, as well as robots and aircraft actuators.
The ball screw assembly is as shown below and includes a circular shaped groove cut in a helix on the shaft. The ball nut also includes an internal circular shaped groove which matches the shaft groove. The nut is retained in position on the shaft by balls moving within the groove. When the nut rotates relative to the shaft the balls move in one direction along the groove supporting any axial load. When the balls reach one end of the nut they are directed back to the other end via ball guides. The balls are therefore being continuously recirculated.
The recirculated ball screw has the following advantages and disadvantages to the conventional threaded power screws (also called leadscrews):
- High Efficiency - Over 90%
- Predictable life expectancy
- Precise and repeatable movement
- No tendency for slip-stick
- Minimum thermal effects
- Easily preloaded to eliminate backlash-with minimum friction penalty
- Smoother movement over full travel range
- Smaller size for same load
- Can be used to create rotary motion from linear motion
- More expensive of course!
- Requires high quality lubrication
- Tendency to move - (Requires additional brakes if locking is required)
- Susceptible to contamination
- (minor) For the same capacity ball screws are not as rigid as conventional power screw (elasticity of balls) but this is usually trivial compared to shaft elasticity.
Another type of low friction screw is the roller screw. This is basically a planetary gear that travels along the main shaft as it spins. The nut includes a number of special threaded rollers arranged around around the central screw, where each roller takes part of the load. This system is efficient and can withstand high loads.
Winches and Hoist equipment
What is the difference between a Hoist and a Winch? Generally, a Hoist is for lifting and a winch is for pulling.
A Winch: Geared for pulling a load on a relatively level surface. It uses a dynamic brake that must slide - e.g. worm gear.
A Hoist: Geared to lift dead weight and needs a locking brake that can support a “hanging” load. If a Hoist can lift (dead weight) 250 kg., then it may be capable of pulling 1000 kg rolling weight across a hard packed surface. If a Winch can pull 1000 kg. across a hard packed surface, it may only have the capacity to support 100 kg. (dead weight) because the winch employs a different braking system than that of a hoist.
Drum Calculator. Use this in conjunction with Drum_Definitions.
Typical safety factor for design of ALL components in a winch or hoist: Safety Factor between 3 to 5.
Winches and hoists can also be hydraulic if the travel distance is suitable. In the building services industry, hydraulics is only an economically viable way to drive an elevator for 2 or 3 floors. Beyond that, a cable drive is essential.
6 x 19S (9+9+1)6 x 36 IWRC
Springs are usually made of spring steel - typically a high carbon steel that may be heat treated to increase the elastic range (high yield point). Smaller springs are made from a steel that is ductile enough to be cold formed. Electrical applications may require springs made from hard brass or copper alloys with spring properties (high yield point). For corrosive environments, stainless steel springs can be used.
We will limit ourselves to the classification of tension/compression/torsion/bending springs, and some of the general principles of helical springs. The performance (i.e.the stiffness and stroke) of a helical spring is determined by wire diameter d, coil diameter D and the number of coils or turns n.
For a given load:
- Increasing the number of coils does not effect stress but it reduces the stiffness.
- Increasing the wire diameter decreases stress and increases stiffness.
- Increasing coil diameter increases stress and decreases stiffness.
So a typical design process works like this. Determine the desired min & max load and travel. Of the 3 parameters, pick a large coil diameter which will determine the wire diameter. The number of coils is determined by the required stiffness. Alternatively, you may have a limitation of a parameter - like coil diameter for example. There is more than one spring that could do the same job.
The following notes are for the design of helical springs - which is more advanced than what we require in this topic, but shows the exact relationship between the 3 design parameters.
1. Springs (Copy of webpage from University of WA - to ensure permanent link)
2. Springs.pdf (Mechanical Design Data Manual TAFE 2000)
Helical Spring Calculator: http://www.calculatoredge.com/mech/hspring.htm