Item 0767

DESIGN: SynchroLite ~ Rotor - Hub - 2-blade Teetering - Bearings - Pitch

Outside Helicopters:

Mini 500 (Talon) Revolution Helicopter Corp.

Centrifugal (centripetal) Load on Hub from 1 Blade:

The centripetal force F acting on a body of mass m is given by the equation F = (mv2)/r, where v is its velocity and r is the radius of its path.

F = (mv2)/r [metric.] Distance r in meters. Mass m in kilograms. Velocity v in meters/sec.

Calculations are based on the following:

Mode is Hover; Rotor speed = 546 RPM; Blade weight = 16.6 lbs.; Tip weight = 1 lbs.; Blade length = 9 feet; Rotor radius = 9' - 7"

Element

Radius (inch)

(meter)

Weight (lb)

(kilo)

Velocity (f/s)

(m/s)

Force (N)

(lb)

1

7" + 5.4" = 12.4"

0.315

1.66

0.753

59

17.98

773

 

2

12.4" + 10.8 = 23.2"

0.589

1.66

0.753

111

33.83

1463

 

3

12.4" + 21.6 = 34.0"

0.864

1.66

0.753

162

49.38

2125

 

4

12.4" + 32.4 = 44.8"

1.138

1.66

0.753

214

65.23

2815

 

5

12.4" + 43.2 = 55.6"

1.412

1.66

0.753

265

80.77

3479

 

6

12.4" + 52.0 = 66.4"

1.687

1.66

0.753

316

96.32

4141

 

7

12.4" + 62.8 = 77.2"

1.961

1.66

0.753

368

112.17

4831

 

8

12.4" + 73.6 = 88.0"

2.235

1.66

0.753

419

127.71

5495

 

9

12.4" + 84.4 = 98.8"

2.510

1.66

0.753

471

143.56

6183

 

10

12.4" + 95.2 = 109.6"

2.784

1.66

0.753

522

159.11

6847

 

Tip weight

115"

2.921

1.0

0.454

548

167.03

4336

 

Total

 

 

17.6

 

 

 

42488

9552

 

Running [FORM: Rotor - Hub Pitch Bearing] with above weight substituted for Vortech's blade, nose, root cap, tip cap and grip the [Actual Axial Bearing Load] is 9,545 pounds and the [Actual Radial Bearing Load] is 11,524 in-lb??. Also, this does not take the hub's precone into account.

If the rotor speed is reduced from 546 to 535 RPM then the [Actual Axial Bearing Load] is 9,164 pounds.

Revolution literature states 'During flight, the centrifugal (centripetal) force on each blade is 8,810 lbs.

Moment from Thrust:

The thrust is GW / 6 blades = (550 / 6) = 92 lbf

The center of thrust is at 75% of disk radius = 0.75 * 104" = 78"

The moment is therefore 92 * 78 = 7,176 lbf-in = 811 N-m.

Moment from Engine Torque:

188 lbf-ft = 254 N-m per rotor divided by 2 blades = 94 lbf-ft = 1,128 lbf-in = 127 N-m.

The Combined Moment from Thrust and Engine Torque:

√((811)2 + (127)2 ) = 1727 N-m

Type and number of bearings.

from David Waddington who posted the [original cone setup] print

"The bearings used were:- 2 off SKF 7207 BEGAP MATCHED PAIR ONE SPACER 0.610" 1 OFF SKF ATN9/C3 WHEN LOCKED TOGETHER THIS STACK GAVE THE CORRECT LENGTH AND PRELOAD. The rotor core was bored internally to a depth of 3.407" BY REVOLUTION so you have to assemble the stack and measure the external size over the bearings and adjust the spacer to size plus .001" more. ( Being angular contact bearings the way you fit to the shaft ie, back to back or face to face gives the strength and increased radial/axial loading)see manufactures recommendations."

From SKF catalog page 286. The 'GA' stands for 'light pre-load'. The 'P' stand for ???

-------------

From SKF catalog page 300. Bearing pair 7207 BECB have Basic dynamic load rating of 11,407 lbs., a basic static load rating of 9,337 lbs and a Fatigue load limit of 396 lbs. The contact angle is 40º.

Assuming that, Revolution's 8.810 lbs of centrifugal (centripetal) force is correct, then the [basic static load rating ~to~ centrifugal (centripetal) load] is (9337 / 8810) = 1.06. This factor is used below when considering bearings for the SynchroLite.

Spacer:

The distance from the center between the 2 angular contact bearings to the centerline of the single radial bearing scales at 1.98".

Bell Helicopters:

From Mr. Trevor Castle, instructor at BCIT

Bell helicopters first used oil in their feathering bearings. The problem with this was leakage and the necessity of high friction at the seals. They then went to grease but cavitation was a problem. The smaller helicopters then went back to oil and the bigger ones went to elastomeric.

Most of the manufacturers now use elastomeric feathering bearings on their larger helicopters.

Ultrasport 254:

Pitch bearings are caged roller and are packed with grease. Axial loads taken by tension-torsion strap.

Helicycle:

"The second reason the rotor system is smooth has to do with its elastomeric feathering bearings." ~ B.J. Schramm

SynchroLite:

FORM: Rotor - Hub - Pitch Bearings:

When the Over Speed in the above form is set to zero, the Actual Axial Bearing Load (Centrifugal) (centripetal): [Fa] value is 3,565 lbs. This is with the composite VR-7 blades and a tip weight of 0.75 lbs. If the tip weight is 1 lb. then the Actual Axial Bearing Load (Centrifugal) (centripetal): [Fa] becomes 3,831 lbs. The other three results do not change.

If heavier blades, such as Vortech extruded aluminum ones, are used for initial testing then the pitch bearings etc. may have to be stronger than the desired strength. The stiffer blades may require slightly stronger pitch bearings.

The radial load on the pitch bearing assemblage will not be the moment; based on the lift plus the drive torque, since the pre-coning angle and centrifugal (centripetal) force will remove most of any moment caused by the lift.

Check to see if radial load is both torque and thrust.

If the hubs preset coning angle is set to hover at gross weight and rotor RPM of 100% then when operating under these conditions, the Actual Radial Bearing Load [Fr] will only be the result of the drive torque.

How does oil get to upper balls? Also, does centrifugal (centripetal) force draw oil outward and away from bearings?

Design Considerations:

The outer bearing can float and is only subject to radial loading.

Go with all ball bearing since the friction is less that roller or needle bearings.

Lubricate with oil, not grease, to minimize friction. Oil will require seals with higher friction. May need reservoir, sit gauge (or side fill hole). How does oil get to upper portion of bearings?

Bearing Selection:

Bearing selection based on Mini 500:

If the Mini 500 pitch bearing series and arrangement was to be used in the SynchroLite then from SKF catalog page 300, then bearing pair 7204 BECB will give a comparable ratio, if copposite blades are used. The basic dynamic load rating of 5,148 lbs., the basic static load rating of 3,731 lbs and the fatigue load limit of 160 lbs. This is comparable for the lighter composite blade and a little too light for the heavier Vortech extruded aluminum blade. Note that the Mini 500's outer single row bearing was replaced by a double row bearing. (see pages in Rotor binder)

It would be preferable to use the 7205 BECB pair, particularly if the gap between the outer bearing and the two inner bearings is very small. They are 23% stronger and the bearing ID (yoke shaft) will increase from 0.7874" to 0.9842". October 14, 2001 ~ This will result in reducing the yoke diameter down from 0.18101" (30mm) to 0.9842" (25mm), which is lighter then current design but still better than the Mini 500.

 

Manufacture's Nbr.

Basic Load Rating for pair

Basic static load rating / centrifugal (centripetal) load *

 

 

Dynamic (lbs)

Static (lbs)

3/4 lb. tip weight ~ 3565

1 lb. tip weight ~ 3831

Angular Contact Bearing:

SKF 7204 BECB (0988)

5148

3732

1.05

0.97

 

SKF 7205 BECB

5643

4586

1.29

1.20

* see comparison with Mini 500 above.

Radial Bearing:
 

For other item on 0088

 

 

 

SKF 6204

2855

1472

Ball Bearing

SKF 6205

3147

1753

Ball Bearing

 

For Item 0306:

 

 

 

 

Torrington MM 203 K-CR

2400

930

 

SKF NUP 203 EC

3867

3215

Cylindrical Roller Bearing

For a comperable spacing to the Mini 500, the distance from the center between the 2 angular contact bearings to the centerline of the single radial bearing scales should be (3565 / 8810) x 1.98" = 0.8012", which is slightly less then having no spacer at all. Just remember that this was the old Mini 500 arrangement.

On the SynchroLite the distance between the radial bearing and the two angular contact bearings can be greater than on the Mini 500 because there is no desire to have the blade root close to the mast. This will reduce the radial load and may allow the use of the SKF 7204 bearings, even with the 1-lb tip weight. This space will also allow for oil entry and measurement.

Additional information: OTHER: Mechanical - Bearing - Elastomeric - General

Additional information: OTHER: Mechanical - Bearings

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Last Revised: November 7, 2004