Selecting a Change Nut
Dynamic Permissible Torque T and Dynamic Permissible Thrust F
The dynamic permissible torque (T) and the dynamic permissible thrust (F) are the torque and the thrust at which the contact surface pressure on the tooth surface of the bearing is 9.8 N/mm2. These values are used as a measuring stick for the strength of the chang 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 change nut. The pV value varies also according to the lubrication conditions.
fS : Safety Factor
To calculate a load applied to the change nut, it is necessary to accurately obtain the effect of the inertia that changes with the weight and the 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 change 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 torque (T) and the dynamic permissible thrust (F) by the corresponding temperature factor indicated in Fig.2 .
Note) In the case of a miniature Change Nut, be sure to use it at 60℃ or below.
Accordingly, when selecting a change nut, the following equations need to be met in terms of its strength.Dynamic permissible torque(T) Dynamic permissible torque(T)
Static permissible thrust(F)
| fS | Static safety factor (see Table1) |
|---|---|
| fT | Temperature factor (see Fig.2 ) |
| T | Dynamic permissible torque (N･m) |
| PT | Applied torque (N･m) |
| F | Dynamic permissible thrust (N) |
| PF | Axial load (N) |
Hardness of the Surface and Wear Resistance
The hardness of the shaft significantly affects the wear resistance of the change nut. If thehardness 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 surface hardness of 250 HV or greater, through hardening as a result of rolling, and surface roughness of Ra0.20 or less. Thus, the dedicated rolled shaft is highly wear resistant.
Calculating the Contact Surface Pressure p
The value of "p" is obtained as followed.
If an axial load is applied:
| p | Contact surface pressure on the tooth from an axial load (PF) (N/mm2) |
|---|---|
| F | Dynamic permissible thrust (N) |
| PF | Axial load (N) |
If a torque is applied:
| p | Contact surface pressure on the tooth under a load torque (PT) (N/mm2) |
|---|---|
| T | Dynamic permissible torque (N･m) |
| PT | Applied torque (N･m) |
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) |
