Category Archives: Linear Motion Design Considerations

Ball Screw Jack vs. Acme Screw Jack

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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.

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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.

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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, brake motor size and ball screw life. To properly size your jack for these constraints, application information must be collected.

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All Your Equations In One Place

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When a linear motion solution is in place & running, it’s easy for the casual observer to think it looks easy. That’s what our industry is all about; making the difficult or even impossible look easy.

But what few people outside the industry realize is just how much work needs to go into the design of a successful linear motion system. The old adage “Measure twice, cut once” doesn’t even begin to cover all the variables, that have to be dealt with.

Now, a new app looks to make that successful design at least a little easier. The Design Guide Pro not only offers selectors for Bevel Gears, Worm Gear Jacks, Bearings and Electric Cylinders, but it also has a calculator section. Here, you’ll find tools to help you establish Energy, Critical Speed, Column Load, Torque and Nut Life as well as helping with Unit Conversions.

All in the palm of your hand. Check it out today by clicking here.

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4 Reasons Why Your Ball Screw Assembly May Fail

Premature failure may be caused by any of the following:

  1. Misalignment of the ball nut to the screw which results in side loading or eccentric loading will reduce life.  Additionally, this may cause the ball bearings to split or get flats on them.  The ball bearings may even break out of the return tubes.
  2. Metal chips, dirt or debris in the ball nut may impede the call bearings from free circulation. The ball bearings may get flats on them because of skidding,
  3. Lack of lubrication.  Proper lubrication will help dissipate heat and reduce metal-to-metal wear of components.
  4. Exceeding the ball nut critical speed can break pickup fingers off, causing the balls to come out of the nut.  Exceeding the screw critical speed will cause the whip resulting in misalignment.

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