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When considering the vast majority of applications in which machine screws are used, it’s important to review the functions of some of the major types of screws. Below, we’ll take a look at the designs, functions and more while we define acme, ball and planetary screws.
The acme screw thread, sometimes referred to as the trapezoidal thread, is used for lead screws. They are often needed for large loads, or when the environment is less than desirable.
The acme thread form has been around for over a century, replacing square thread screws which had straight-sided flanks and were difficult to manufacture.
There are two main classes of acme thread forms: general purpose (G) and centralizing (C). The general purpose and centralizing thread forms have a nominal depth of thread of 0.50 x pitch and have a 29 degree included thread angle, which has allowed companies to develop unique screw diameters and leads. European metric Trapezoidal thread forms have a 30 degree Included thread angle.
When compared to general-purpose thread forms, centralizing threads are manufactured with tighter tolerances and reduced clearance on the major diameter. For instance; If an acme nut is side loaded with a radial load, a “G” class will wedge when the nut thread flanks come in contact with the screw thread flanks. To prevent this wedging, a “C” class thread form can be used, since it utilizes less clearance and tighter tolerances are allowed between the major diameter of the nut and the major diameter of the screw.
Industry leaders have developed several unique thread forms, such as stub acme forms and 40 degree included angle, which allow them to provide a variety of diameter and lead combinations.
For loads requiring a greater amount of efficiency, companies often turn to ball screws. A ball screw assembly is a device comprised of a nut, screw, and reciprocating ball bearings. The bearings provide the thread engagement between the nut and screw.
Ball screws offer an efficient means for converting rotary motion to linear motion. A ball screw is an improvement over an acme screw just as an anti-friction ball bearing is an improvement over a plain bushing.
In the long run, ball screw systems can prove to be a cost-effective alternative to pneumatic or hydraulic systems, which require constant electrical and air power.
Planetary Roller Screws:
Planetary roller screws are remarkable devices designed to convert rotary motion into axial force or vice versa.
The planetary roller screw design offers multiple advantages and reliability for the most demanding applications when compared with other lead screw types due to its rolling motion. These screws offer high efficiency even in relatively shallow lead designs.
The multitude of contact points can carry large loads and provide very high resolution (small axial movement) when using very shallow leads. Planetary roller screws produce high rotational speeds with faster acceleration without adverse effects.
Tips for how to select the right one for your application
Worm Gear Screws Jacks can provide long duty life, high load capacity and flexible design. They come in two major categories, Ball Screw and Machine Screw. In this post, we hope to help you identify the best type for your application.
Ball Screw Jacks use a ball screw and nut made from hardened alloy steel with bearing balls carrying the load between nut & screw. This rolling action reduces the friction between nut and screw, permitting smooth and efficient load movement that requires approximately 1/3 less torque than a machine screw jack with the same load.
Machine Screw Jacks incorporate an alloy or sometimes stainless steel worm which drives a high strength bronze worm gear, or drive sleeve. The worm shaft is supported on anti-friction tapered roller bearings with external seals that prevent lubrication loss. The drive sleeve can also be supported on tapered roller bearings, or ball thrust bearings. Rotation of the drive sleeve causes the acme thread lifting screw to translate or rotate, depending on the jack configuration.
Because of their efficiency and lower power requirements, Ball Screw Jacks are often preferred. However, several factors can make Machine Screw Jacks preferable. For quick reference …
Machine Screw Jacks are best used for:
• Resistance to backdriving
• Environments with vibration
• Manual operation
• High static loads
• Corrosion resistance (with stainless steel versions)
Ball Screw Jacks are preferred for:
• Long travel lengths
• Long, predictable life
• High duty cycles
• Oscillating motion
Both types can be metric or inch, come in several types (Upright, Inverted, Upright Rotating and Inverted Rotating) and multiple jacks can be laid out in H, U, T and In-Line arrangement.
You can also employ multiple jacks in tandem, depending on the physical design and size of the equipment, its stiffness and the guide system. This will, however, introduce challenges with drive, alignment and synchronization.
Any jack system is limited by multiple constraints: load capacity, duty cycle, horsepower, column strength, critical speed, type of guidance, brakemotor size and ball screw life. To properly size your jack for these constraints, application information must be collected.