jason tarrant / november 2008 mice target mechanical design l contents »shaft bearing redesign...

Jason Tarrant / November 2008

MICE Target Mechanical Design Contents

» Shaft Bearing Redesign– Cross section shape– Anti-rotation device and location– Target shape and attachment– Bearing design– Assembly issues with alternative shaft and bearings

» Assembly for ISIS



MICE Target Mechanical Design Cross section shape

» Current cruciform shape– ‘+’ or ‘x’ same stiffness– For 11mm2 CSA I = ~18mm4

– For a round shaft with same CSA @ 6mm OD I = 39mm4 > 2 x as stiff.



MICE Target Mechanical Design Alternative Round or Square?


RoundFor a 6mm OD tube with a CSA of 11mm2 the ID works out to be = ~4.7mm. Calculating the second moment of area (I) = ~39mm4. For a 0.5mm wall the OD will be 7.5mm to give 11mm2 cross section, for this I = ~68mm4.

SquareFor a square tube the cross corner dim will be ~8.5mm for a square section that has an across flats dim of 6mm. For the square section with an A/F of 6mm the A/F of the ID will be 5mm (0.5mm wall) and the I will be ~56mm4 in both the horizontal/vertical and 45deg orientations, however overall it does fall outside 6mm. For a shaft with across corner measurement of 6mm I = 23mm4.


MICE Target Mechanical Design

Alternative Round or Square?» Round advantages

– Various standard sizes– Easier to finish a round shaft and produce hole to

tight tolerances / fine finish– Techniques for joining (threads, orbital welding)

easier than square

» Round disadvantages– Square shaft easier to control rotation



MICE Target Mechanical Design Anti-rotation device

» Placement of feature (assume slot or flat on shaft)

– Tubular section advantages Stiffer than solid top section, especially with feature Wider flat (but split) and wider deeper slot = more able to

resist rotation thus preventing potential misalignment or even lock-up

– Tubular section disadvantages Cutting through tubular wall may cause relaxation of

residual stress and misshapenness of shaft (follower / sprung pad cannot be part of bearing if tube is misshapen)




» Viable types– Slot and key

Key on shaft relies on strength of connection to shaft to prevent it parting, issues with weight & balance

Slot on shaft saves weight, key mounted elsewhere can be given a very strong connection

– Flat and follower Flat loaded with a sprung pad

(could be at a bearing) Flat loaded with a thin sprung skid

– Others? Jason Tarrant / November 2008



» Slot & key advantages– More positive rotation prevention, especially opposed to

a narrow flat– A key should not impose transverse load– If shaft motion is accurately restrained by bearings the

key can be sprung or loosened to minimise force required for vertical motion

– Easier to machine, less operations, no feathered edges– Section is marginally stiffer & shaft more balanced (less

material removed)– Might also be used as end stop for dropped shaft




» Slot and key disadvantages– Tight fit of key might be required to prevent

hammering that is assumed to be happening with bearings

– Coating the slot (or coating QA) might be difficult as the face will not be exposed (possible 1.5mm wide)




» Possible location– Shaft movement keeps permanent

magnet within stator = possible length to keep slot inside w/o passing thro bearing? Move bearing?

– Key, part of stator tube or similar components?

– If flexible key, could be vertical cantilever



Target Shape» Round tubular end as target» Formed flat target from tubular end» Separate component of any shape

attached to round section that requires attachment…

Target attachment» Threaded, pinned & welded




Bearing Function» Allow constrained vertical movement

– Keep permanent magnet aligned with stator– Target tip in beam– Optical grating w.r.t optical sender / receiver

» Minimise free horizontal movement of shaft – To prevent rattling / hammering

» Minimise driving force– Allow low driving force of shaft– No frictional heating



Requirements w.r.t main functions» Allow constrained vertical movement

– Coaxiality of bearings with assembly components (x,y & orientation), especially stator.

» Minimise free horizontal movement of shaft – Minimal clearance between bearings and shaft

» Minimise driving force– Low friction between shaft and bearing– No transverse force on shaft by bearings (good

location and fit)



Other essential requirements» Durable

– For required life of target

» Compatible– Radiation– Non-magnetic– Compatible with material/coating of shaft– Compatible with adjacent part materials



Possible supplementary functions» Prevent rotation of the shaft» Damping to prevent vibration issues» End-stop for a dropped shaft


MICE Target Mechanical Design Solutions for ‘main function’ requirements

» Constrained vertical movement– Align bearings with Target assembly

Machine separately and assemble to accurately m/c assembly (current alignment of components?)

Machine separately and assemble into assembly that allows adjustment, e.g. a gimbal type + x,y mount.

Part machine and fit to assembly / sub assembly then final machine in ‘one-shot’ (sub assembly important)

At least one bearing flexibly mounted to take up misalignment (even dynamic misalignment)



» Minimise free horizontal movement– Reducing the clearance between shaft and bearing

Alternative bearing hole shape that is easier to machine smooth and to tight tolerances (requires shaft change too)

Ensure that the shaft is as accurate as possible (straight, smooth, tight tolerances)

Decrease bearing misalignment (see previous) Flexible bearing to give snug fit Sprung device that keeps shaft in contact with bearings



» Minimise driving force– Minimise Friction

Low friction coating (hard wearing, not necessarily hard) Fine surface finish (ground, polished, honed)

– Minimise transverse force Bearing alignment (see previous) Bearing clearance (see previous)

– Also minimise mass in shaft



» Assembly issues with round shaft & slot/key– Passing target through bearing– Engaging key in slot– Other….


MICE Target Mechanical Design Assembly for ISIS» 3 Choices

– 1/ R78 after long test, inspect, predict life, install in ISIS if prediction OK, problems/issues?

– 2/ R78 refurbished, shaft and bearings from same batch as test model, problems/issues?

– 3/ Parallel identical assembly made at same time as R78 test model, problems/issues?


MICE Target Mechanical Design Assembly for ISIS - considerations

Question R78 inspect & use R78 refurbished Parallel Identical

How alike is the model being installed to the test model?

It is the test model Minimum difference in all components to test model (easy to be ~ identical)

Many new components = biggest difference (can it really be made identical?)

Is it known to work? Known to work for reasonable no. of cycles

Expected to work based on previous test, potentially only shaft bearing issues

Expected to work based on R78 model but potential for many differences (shaft, bearing, driving, electrics)

What components are worn?

Potential for wear hence reduced life throughout (or is it run-in?)

Shaft & bearings replaced but still many components part worn

All components fresh

Expected life? Can shaft, bearing, drive wear/damage be gauged?

Can drive be judged for wear/damage?

Relies on being same as the R78 offline model

Other Implications? -How fast can the offline target produce ‘x’ cycles?-How identical is the offline model? Could be a reverse problem to using parallel identical model in ISIS.

-Sensible Test/Run-in?-Offline model identical, could be a reverse problem.

-Sensible Test/Run-in?

jason tarrant / november 2008 mice target mechanical design l contents »shaft bearing redesign...

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