Selecting a Lead Screw Nut
Permissible Dynamic Thrust
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 9.8N/mm2 This value indicates the strength of the Screw Nut.
pV Value
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 (fS) shown in Table1 .
| Type of load | Lower limit of fS |
|---|---|
| For a static load less frequently used | 1 to 2 |
| For an ordinary single-directional load | 2 to 3 |
| For a load accompanied by vibrations/impact | 4 or greater |
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 temperature factor indicated in Fig.2 .
Accordingly, when selecting a lead screw nut, the following equations need to be met in terms of its strength.
Static permissible thrust(F)
| fS | Safety factor (see Table1) |
|---|---|
| fT | Temperature factor (see Fig.2 ) |
| F | Dynamic permissible thrust (N) |
| PF | Axial load (N) |
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, the abrasion loss increases as indicated in Fig.3 . The roughness of the surface should preferably be Ra0.80 or less.
A special rolled shaft achieves the surface hardness of 250 HV or greater, through hardeningas a result of rolling, and surface roughness of Ra0.20 or less. Thererfore, the dedicated rolled shaft is highly wear resistant.
Calculating the Contact Surface Pressure p
The value of "p" is obtained as followed.
| p | Contact surface pressure on the tooth from an axial load (PFN) (N/mm2) |
|---|---|
| F | Dynamic permissible thrust (N) |
| PF | Axial load (N) |
Calculating the Sliding Speed V on the Teeth
The value of "V" is obtained as followed.
| V | Sliding speed (m/min) |
|---|---|
| Do | Effective diameter (see specification table) (mm) |
| n | Revolutions per minute (min-1) |
| S | Feeding speed (m/min) |
|---|---|
| R | Lead (mm) |
| α | Lead angle (see specification table) (°) |
