1. GENERAL CONSIDERATIONS

2. DRIVING UNITS DESCRIPTION AND DIMENSIONING
2.1. - Spider operation with telescope at horizon pointing

3. M2 F4 STRUCTURAL JACK SUPPORT

The current design of the rotation and preload mechanism has been conceived to be operated with the telescope zenith pointing only. Consequently the rotation movement requires very little power, being the c.o.g. displaced along an horizontal path. The motor and gear selection has been dictated by the amount of preload to be applied at the spider joints.
For these reasons and also for the obstruction constraints towards the rolling sector box section, the levers geometry is very unfavourable when the spider is in the rest position, that means having for the max. push-pull force exerted by the screw-jack a very small rotation torque applied to the spider axis.
The normal maintenance operations on the mirrors and related supports shall be performed with the aid of movable lifting platforms when the telescope is horizon pointing and the interested spider in the rest position.
In such a situation it is not possible to modify the orientation of the same without repositioning the telescope zenith pointing. On the contrary it is possible, with the telescope in maintenance mode and horizon pointing, rotating a spider from its work position toward the rest one as far as the demanded motor torque reaches the max. torque calculated for the preload of the spider joints.
This means approximately that the spiders M2 / F15 and M3 could be rotated of about 60° ( on a total rotation angle of 76° ) and the M2 / F4 of not much than 25° .

For the above reasons and remarking that when the spider is in the rest position its weight is transmitted to the rolling sector through the hinge supports and the end stops fixed to the rear bars, the driving units have been dimensioned with the same gear / motor unit for the three spiders, modifying the screw-jack size only according to the max. preload to be applied at the spider itself.

The Driving Mechanisms consist in :

Assumed the overall efficiency of the transmission is h = 0,22, the force needed at the screw axis for the loading of the Belleville springs is equivalent to a demanded torque at the motor axis of:

M2 / F15 - C mot =

M3 - C mot =

M2/F4 - C mot =

The selected motor is rated 5.5 kW. About the 15% of the nominal power is consumed in the worst case by the inverter (ref. motor dealer). Moreover, the available power must be scaled down to account for the altitude: at 3250 mt the factor is 0.80 (ref. motor dealer).

Thus, the available power is , so there is sufficient margin to adjust the preload force up 20.5 x 1,5 = 31 Nm according to the final mass of the opto-mechanics and instruments.

The max input torque for the K210 screw is computed by neglecting the power scaling due to the inverter . This value is about 10% beyond the max allowable one that is 132 Nm, but can be considered acceptable given the screw own safety margins.

The nominal travelling speed of the screw is:

and the corresponding rotating time is approximately

preload at reduced speed = 2 min

On field shall be selected the most suitable travelling speed, that can be trough the inverter at the 150% of the nominal, reducing the positioning time of the spider at less than 1,5 min.

When the spider is not operated it has to be parked by pre-loading the rear bars innermost mechanical end stops. In fact, this assures that when the telescope is horizon pointing the unbalancing torque is reacted by the two rear bars, thus not loading the screw mechanism.

For the M2 / F15 unit the spider own weight is 9780 N (c.o.g 2423 mm from the spider tip) and the hub one is 5500 N.

In the rest configuration with the telescope at horizon the overall torque is M₀ = 37.4 KNm, where the point "O" is the support "H-I"(drawing 541a011).

If this torque is correctly reacted by the two rear bars end stops, their load is about 6.3 KN.
On the other hand, if the end stops are not engaged the reaction load on the screw would reach the 383 KN.

This value is far beyond the K210 screw limit load that is 100 KN and would cause the damage of the screw and its hinge on the sector.

Scope of the present analysis is to minimize the structural deflection of the composed beam on which the actuator jack is mounted on.

The deflection of the support modifies the forces layout giving larger reacting forces on the structure and an extra-stress on the material.

The first solution earlier proposed showed a displacement of 17 mm on the plane of the jack action; it consisted in the superposition of a rigid rotation - » 12 mm - and a deflection of the bar - » 5 mm - as it can be seen from the two following plot.

This behaviour was due to the position of the inner attachment plates, which rotated following the deflection of the bar:
In doing so the outer attachment plates rotated also, causing a torsional load on the bar itself.

The maximum stress according to the Von Mises criterion is about of 300 N/mm2.

F1 = 58'500 N x Þ constrain
F2 = 39'700 N

In order to reduce the displacement of the bar, the inner attachment plates were moved toward the outer ones; therefore the maximum displacement in this condition is less than 4 mm, mainly due to the deflection of the bar (see next pictures).

The maximum stress, according the Von Mises formulation, is about 180 N/mm2.