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Types of Threads

The acme thread form, established over 100 years ago, replaced square thread screw, 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. Metric thread forms have a 60° 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.

Technical Definitions

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.

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.

MAJOR DIAMETER

The outside diameter of the screw.

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

SIDE LOAD

A load that is applied radially to the nut.

LEAD ACCURACY

Lead accuracy is the difference between the actual distance traveled versus the theoretical distance traveled based on lead.

SCREW STARTS

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

Lead Screw Selection

Permissible Dynamic Thrust and pV Value

The permissible dynamic thrust (F) is the level of thrust at which the contact surface pressure exerted by the bearing on the screw tooth surface is 9.8 N/mm2.
This value indicates the strength of Screw Nut.

With a sliding bearing, a pV value, which is the product of the contact surface pressure (p) and the sliding speed (V), is used as a measuring stick to judge whether the assumed model can be used. Use the corresponding pV value indicated in
Fig.1 as a guide for selecting a lead screw nut. The pV value varies also according to the lubrication conditions.

fS : Safety Factor

To calculate a load applied to the lead screw nut, it is necessary to accurately obtain the effect of the inertia that changes with the weight and dynamic speed of an object. In general, with the reciprocating or the rotating machines, it is not easy to accurately obtain all the factors such
as the effect of the start and stop, which are always repeated. Therefore, if the actual load cannot
be obtained, it is necessary to select a bearing while taking into account the empirically obtained safety factors shown in Table1 .

fT: Temperature Factor

If the temperature of the screw nut exceeds the normal temperature range, the seizure resistance of the nut and the strength of the material will decrease. Therefore, it is necessary to multiply the dynamic permissible thrust (F) by the corresponding temp. factor indicated in Fig.2 .

Static permissible thrust(F)

Accordingly, when selecting a lead screw nut,
the following equations need to be met in terms of its strength.

Hardness of the Surface and the Wear Resistance

The hardness of the shaft significantly affects the wear resistance of the lead screw nut. If the hardness is equal to or less than 250 HV, abrasion loss increases as indicated in Fig.3 . The roughness of surface should preferably be 0.80a or less. A special rolled shaft achieves the surface hardness of 250 HV or greater, through hardening as a result of rolling, and surface roughness of 0.20a or less. Thererfore, the dedicated rolled shaft is highly wear resistant

HARDNESS OF THE MATING SURFACE (HV | Apex Leadscrews

Contact Surface Pressure (p)

$Contact Surfrace Pressure | Apex Leadscrews$

Sliding Speed (V) on the Teeth

Efficiency

Thrust(Fa)

Critical Speed

Once the load, speed, length and end fixity are identified, the next factor to consider is the critical speed. The speed that excites the natural frequency of the screw is referred to as the critical speed. Resonance at the natural frequency of the screw will occur regardless of the screw orientation (vertical, horizontal etc.) or if the system is designed so the nut rotates about the screw. The critical speed will vary with the diameter, unsupported length, end fixity and rpm. Since critical speed can also be affected by shaft straightness and assembly alignment, it is recommended that the maximum speed be limited to 80% of the calculated critical speed. The theoretical formula to calculate critical speed in rpm is:

Column Strength

When a screw is loaded in compression, its limit of elastic stability can be exceeded and the screw will fail through bending or buckling. Theoretical formula to calculate the column strength in pounds is:

If the selected lead screw does not meet critical speed and/or compression load criteria, consider the following options:
a) Increase screw lead and reduce rpm
b) Change end fixity (e.g. simple to fixed)
c) Increase screw diameter
d) Design to use screw in tension load

Critical Rotational Speed Limit Values

Every lead screw has a rotational speed limit. This is the point at which the rotational speed sets up heavy vibration. This critical point
changed depending on the end bearing supports used and the bearing
combination.To use this chart, you must determine the speed of
rotation required and the maximum length between the bearing
supports. Then select one of the four bearing combinations shown
below. The critical speed limit can be found by locating the point at
which the speed of rotation (horizontal lines) intersects with the
unsupported shaft length (vertical lines) as modified by the bearing
combination listed below. It is recommended that the lead screws be operated at no more than 80% of the critical speed limit value.

Warning: The graphs for the shaft diameters illustrated are based on the smallest minor diameter of a standard shaft within the nominal size range & are cut off at the maximum speed of rotation for the nut. Value for the rotational speed MAY NOT BE EXCEEDED, whatever the shaft length.

Critical Buckling Force Limit Value

This graph is used to determine the maximum compression loading on the shafts. Normally, shafts operated under tensile stress are capable of withstanding a loading up to the design load capacity of the nut. The bearing combinations influence the load capacity of the shaft. The four standard variants are listed below with the corresponding bearing scenarios. To determine the safe minimum diameter of the shaft, you must determine the point at which the graphs for the
compressive load (horizontal) and the shaft length (vertical) intersect.

Critical Buckling Force limti Value | Apex Leadscrews

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Technical Documentation

Types of Threads

Technical Definitions

STATIC LOAD

DYNAMIC LOAD

PV LOAD

TENSION LOAD

COMPRESSION LOAD

THRUST LOAD

OVERTURNING LOAD

SIDE LOAD

MAJOR DIAMETER

PITCH DIAMETER

ROOT (MINOR) DIAMETER

PITCH

LEAD

SIDE LOAD

LEAD ACCURACY

SCREW STARTS

Lead Screw Selection

Permissible Dynamic Thrust and pV Value

fS : Safety Factor

fT: Temperature Factor

Static permissible thrust(F)

Hardness of the Surface and the Wear Resistance

Contact Surface Pressure (p)

Sliding Speed (V) on the Teeth

Efficiency

Thrust(Fa)

Critical Speed

Column Strength

Critical Rotational Speed Limit Values

Critical Buckling Force Limit Value

Contact Us

Address

Contacts

Links

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