Load cycle definition:

1 full cycle is composed by a back and a forth movement. 
The cycle description shall enclose the back and a forth movements. 
The cycle is splited into different steps. For each step the displacement, 
the load and the speed shall be entered. 
If the load varies within a step (e.g. load 4), the mean load 
within this step shall be calculated using the formula: 
Fmean = (Fmin + 2Fmax) / 3 with "Fmin" and "Fmax" in absolute values. 
The desription of the load cycle permits the calculation of 
the cycle mean load. 
This mean load value is used to determine the required dynamic load 
carrying capacity of the product.
						

Axial play / Backlash elimination / Max rigidity:

Axial play - Backlash:
Ball or roller screws with backlash are the most standard ones.
At the load reversal, the axial play will induce a few hundredth of milimeters of free displacement.
Backlash elimination:
To increase positionning precision at load reversal, the backlash (free displacement) can be eliminated.
It can be done on several ways:
- with oversized balls : BD, BX and BN ball screws.
- with oversized rollers : BR and BV roller screws.
- with internal preload : TL ball screws.
Max rigidity / Preload:
To increase the product stiffness and positionning precision under low load.
The preload is set with a spacer and split nuts for roller screws (PRU, PRK, PVU, PVK) and with nut internal preload for ball screws (PN, PND, PK, PKD)
It gives a high and even stiffness level under low load.

Support bearings types definition:

This is used for critical speed and buckling calculation. It depends on the support bearing setup.
The most common support is:
: fixed - supported, with a thrust bearing unit at one end and a radial bearing at the other end.

We also can have:
: fixed - free, with a thrust bearing unit at one end and free at the other end.
: fixed - fixed, more rare because the risk of internal shaft stresses.
: supported - supported, risk of excessive bending and buckling.

Lead precision definition:

For roller and ground ball screws, the standard lead precision is G5. G3 and G1 are also available on request.
For rolled ball screws, the standard lead precision is G9. G7 and G5 are also available on request.

Load direction definition:

Compression or tension?
When screws are subjected to compression loads, the buckling can be a failure mode and a calculation will be made by the programme.
This buckling calculation uses the Euler formula. The inputs for this calculation are the buckling length of the shaft, the maximum externa load and the ends support type.

When screws are subjected to tensile loads, no special calculation has to be made.

When screws are subjected to both tensile and compression loads, the buckling has to be considered and the calculation will be made considering compression loads.

Dynamic peak load definition:

Not only the mean cycle load has to be considered; in some cases, the dynamic peak load can be a restrictive criteria.
In a first approach the dynamic peak load shall not exceed 80% of the dynamic load carrying capacity of the product.
The value entered as "dynamic peak load" is compared to 80% of the dynamic load carrying capacity (Ca) of the product, and, if necessary, a product with a higher Ca will be proposed.
An advanced study can be made by Ewellix application engineers if required.

Remark: a load is considered as dynamic as soon as there is an axial displacement; even some tenth of mm can be considered as dynamic.

Static peak load definition:

A load is considered as static when the load is applied without any movement.
In case of high static (peak) load, it shall be entered to allow the comparison with the static load carrying capacity (C0a) and the proposition of the appropriate solution.
For roller screws it should not exceed 80% of the product C0a and for ball screws 60%.

Acceleration values:

Rolled ball screws acceleration : Maximum acceleration = 4000 rad/s2.

Roller screws acceleration : Maximum acceleration = 12000 rad/s2.

Nota: those values are not uset to calculate the torques.

Buckling and critical speed length definition:

If longer than the working stroke, this is the length which will be used to calculate the buckling (Euler theorical) and critical speed values.
In that case the working stroke is used to calculate the life time.

Buckling safety factor definition:

The column loading of a screw shaft must be checked when it is subjected to dynamic or static compression loading. The maximum permissible compressive load is calculated using the Euler formulae, with a safety factor of 3. The type of shaft end mounting is critical for selecting the proper coefficient to be used in the Euler formulae. It’s possible to change this value for particular applciations at the users own risk.

Max stroke definition:

to be used in case the user needs to have a longer stroke than the one used in the working cycle.