Preliminary Design Review - Mechanisms Design

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Preliminary Design Review - Mechanisms Design

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Preliminary Design Review - Mechanisms Design -
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iSHELL Preliminary Design Review: Mechanisms
•  Mechanisms Overview:
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iSHELL Preliminary Design Review: Mechanisms

•  High-Level Requirements:

Mechanism CB mirror Gas cell Window cover K-mirror Slit wheel

Temp ~280 K ~280 K ~280 K
75 K 75 K

Type In/out In/out In/out Continuous Detent

Range 2 positions 2 positions 2 positions > 360 degrees 5 positions

Dekker stage SV filter wheel

75 K Detent 75 K Detent

4 positions 15 positions

Order sorter wheel 75 K Detent

10 positions

IG selection mirror XD wheel

75 K In/out 75 K Detent

2 positions 12 positions

XD wheel tilt Spectrograph focus

75 K Continuous ± 5 degrees 75 K Continuous ± 2 mm

Repositioning precision ± 0.5 mm ± 0.5 mm ± 0.5 mm ± 0.1 deg on sky (1 pixel) ± 1 pixel
± 1 pixel ± 0.1 mm
± 0.1 mm
± 0.1 mm ± 1 pixel (15 arcsec)
± 1 pixel (15 arcsec) ± 50 µm

Element size Beam Beam Beam Beam 30 mm diameter 5 mm thick n/a 25 mm diameter ~5 mm thick 6 x 10 mm ~3 mm thick Beam ~32 x 50 mm to ~32 x 40 mm ~ 7 mm thick n/a n/a

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GENERIC BEARING MOUNT
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Athermal Bearing Mount
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Athermal Turret Axle Design
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Athermal Turret Behavior
•  Components contract as turret cools from ambient to operating temperature •  Aluminum turret & axle contract/expand as a rigid body
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Athermal turret axle design
•  Eliminates axial & radial displacement of turret relative to axle due to CTE mismatches between steel bearings and aluminum turret during cool-down/warm-up.
•  Turret/axle assembly (CTE) behaves as though it were a single material as temperature changes
•  Turret remains aligned when warm or cold •  Don’t need to design in “warm” bearing clearances that
makes turret sloppy when warm & tight when cold
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DISCRETE ANGULAR POSITION MECHANISMS (DAPM)
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DAPM: Choice of a type of wheel
Here is a summary of the pros and cons of a Geneva drive versus a compliant worm drive:
•  Pro: Geneva mechanisms locate the wheel close to the required position => less detent force required
•  Pro: Geneva mechanisms require less time to index than a worm drive because the reduction ration is usually less
•  Con: Geneva mechanisms usually require the use of gear reduction. •  Con: due to the reduction and the use of a cam, Geneva mechanisms
have more moving parts than a worm drive •  Pro: Geneva wheels are less expansive to fabricate than worm drives •  Con: Geneva mechanisms are limited in the number of discrete
positions.
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Geneva MechanismsDesign ReviewTurretWheelWorm Drive