LinearLibraryItem - Helix Glossary and Technical Data

Helix Glossary and Technical Data

THREAD TYPES

The acme thread form, established over 100 years ago, replaced square thread screws, which had straight-sided flanks and were difficult to manufacture.

There are three main classes of acme thread forms: general purpose (G), centralizing (C), and stub acme. The General Purpose and Centralizing thread forms have a nominal depth of thread of 0.50 × pitch and have a 29° included thread angle. Trapezoidal thread forms have a 30° included thread angle. Helix precision lead screw assemblies have a 40° angle.

Compared to general-purpose thread forms, centralizing threads are manufactured with tighter tolerances and reduced clearance on the major diameter.

Stub acme threads follow the same basic design, but have a thread depth less than one half the pitch.

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 wedging, less clearance and tighter tolerances are allowed between the major diameter of the nut and the major diameter of the screw.

CAUTION - Although a side load will not cause a centralizing thread to wedge, the nut is not designed to operate with a side load such as a pulley, drive belt, etc.

DEFINITIONS

PITCH DIAMETER - On an acme screw, this diameter is approximately halfway between the land diameter and the root diameter. It is the diameter at which the thread thickness is equal to the space between threads.

ROOT (MINOR) DIAMETER - The diameter of the screw measured at the bottom of the thread.

PITCH - The axial distance between threads. Pitch is equal to the lead in a single start screw.

LEAD - The axial distance the nut advances in one revolution of the screw. The lead is equal to the pitch times the number of starts.

PITCH × STARTS = LEAD

NOTE: Helix precision lead screw designations reference nominal diameter and lead. For example: 250×125 screws advance 0.125" in one revolution and require eight turns for one inch of travel. A 250×125 screw has 2 starts and a 0.062” pitch.

0.062" PITCH × TWO STARTS = 0.125" LEAD

LEAD ACCURACY - Lead accuracy is the difference between the actual distance traveled versus the theoretical distance traveled based on lead. For example: A screw with a 0.5 inch lead and 0.004 inch per foot lead accuracy rotated 24 times, theoretically moves the nut 12 inches.

(24 Revolutions × .500 inches per revolution = 12.000 inches of travel)

With a lead accuracy of .0004”/inch, actual travel could be from 11.996 to 12.004 inches.

SCREW STARTS - The number of independent threads on the screw shaft; example one, two or four.

MATCHED LEAD - When multiple screws are used in unison to move a load with precise synchronicity, screws of similar lead accuracy can be factory selected and supplied as sets. Consult factory for matched lead set tolerances.

STRAIGHTNESS - Although Helix precision lead screws are manufactured from straight, cylindrical material, internal stresses may cause the material to bend or yield.

When ordering random lengths or cut material without end machining, straightening is recommended. Handling or machining of screws can also cause the material to bend or yield. Before, during and after machining, additional straightening is required. When ordering screws with machined ends from Helix Linear Technologies, the following straightness tolerances can be expected: Helix precision rolled and milled lead screws are straight within 0.010 inch/foot and will not exceed 0.030 inch in any 6-foot section when shipped from the factory.

Helix precision ground lead screws are straight within 0.001 inch/foot when shipped from the factory. If tighter straightness tolerances are required, contact Helix customer service.

LIFE - Helix precision lead screws are manufactured from high quality materials with excellent dynamic properties. Because of the variable effects of friction, lubrication and cleanliness, a specific life cannot be predicted. Proper lubrication, regular maintenance, and operation within specified limits will extend the life of lead screws.

EFFICIENCY - Efficiency of Helix precision lead screw assemblies range from 15% to 85%. These efficiencies are dependent upon nut material, lubrication, lead and thread form. The efficiencies for each assembly are listed on the following pages.

BACKDRIVING - Normally, lead screws are used to convert rotary motion into linear motion. Backdriving is the result of the load pushing axially on the screw or nut to create rotary motion. Generally, a nut with efficiency greater than 50% will have a tendency to backdrive. If a self-locking assembly is required, select a nut with efficiency below 35%.

CAUTION: Vibration can cause any lead screw assembly to creep or backdrive. When using lead screws, applications should be analyzed to determine the necessity of a brake, especially when the possibility of injury may occur.

BACKLASH - Backlash (lash) is the relative axial clearance between a screw and nut without rotation of the screw or nut. Backlash information for Helix precision lead screws and nuts is listed within the data section of this catalog. Lash will always increase with use. Helix Linear Technologies has developed several unique ways to reduce or remove the lash between the screw and nut.

STATIC LOAD - The maximum thrust load – including shock – that should be applied to a non-moving nut assembly. Actual maximum static load may be reduced based on end machining and screw mounting hardware.

DYNAMIC LOAD - The maximum recommended thrust load which should be applied to the lead screw and nut assembly while in motion. PV LOAD - Any material which carries a sliding load is limited by heat buildup caused by friction. The factors that affect heat generation rate in an application are the pressure on the nut in pounds per square inch of contact area and the surface velocity in feet per minute at the major diameter. The product of these factors provides a measure of the severity of an application.

TENSION LOAD - A load that tends to “stretch” the screw.

COMPRESSION LOAD - A load that tends to “squeeze” the screw.

THRUST LOAD - A load parallel to and concentric with the axis of the screw.

OVERTURNING LOAD - A load that tends to rotate the nut radially around the longitudinal axis of the screw.

SIDE LOAD - A load that is applied radially to the nut.

Figure 1