Untitled Document

X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml Date: Sun, 08 Oct 2006 17:58:31 -0400 Message-ID: X-Original-Return-Path: Received: from smtp106.sbc.mail.mud.yahoo.com ([68.142.198.205] verified) by logan.com (CommuniGate Pro SMTP 5.1c.5) with SMTP id 1449610 for lml@lancaironline.net; Sun, 08 Oct 2006 13:40:24 -0400 Received-SPF: none receiver=logan.com; client-ip=68.142.198.205; envelope-from=elippse@sbcglobal.net Received: (qmail 86123 invoked from network); 8 Oct 2006 17:40:03 -0000 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=s1024; d=sbcglobal.net; h=Received:Message-ID:From:To:Subject:Date:MIME-Version:Content-Type:X-Priority:X-MSMail-Priority:X-Mailer:X-MimeOLE; b=fZ6XrJFKFqE100WfHQoZJPBCHFN3im5EHndrtr9hMH3ZGS/5a7hqEfSljTGlQsCF4R/ENC6zAV9woH/w/VgtloSH9j5/u7eTBjAtG188oLr/urkfVWwrgbO1fxMlVlAVMOZTyrZi1WEDGNKoDXuajM1KTCi+8hEDsFiM8KJd/J0= ; Received: from unknown (HELO Computerroom) (elippse@sbcglobal.net@75.15.114.170 with login) by smtp106.sbc.mail.mud.yahoo.com with SMTP; 8 Oct 2006 17:40:03 -0000 X-Original-Message-ID: <000701c6eb00$d29a6920$aa720f4b@Computerroom> From: "Paul Lipps" X-Original-To: "Marv Kaye" Subject: Belleville washers X-Original-Date: Sun, 8 Oct 2006 10:40:07 -0700 MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_NextPart_000_0003_01C6EAC6.1F717030" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.2869 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2962 This is a multi-part message in MIME format. ------=_NextPart_000_0003_01C6EAC6.1F717030 Content-Type: multipart/alternative; boundary="----=_NextPart_001_0004_01C6EAC6.1F789C20" ------=_NextPart_001_0004_01C6EAC6.1F789C20 Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Here's Vance Jaqua's article on Belleville washers. Sorry! The diagrams = never came through! ------=_NextPart_001_0004_01C6EAC6.1F789C20 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable

Here's Vance Jaqua's article on Belleville = washers. Sorry!=20 The diagrams never came through!

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BOLTING WOOD PROPS - INCLUDING THE USE OF BELVILLE SPRING WASHERS =

Paul Lipps (an innovative prop designer referenced = in a=20 previous issue of Contact Magazine) contacted me after reading the = treatise on=20 bolt preload. He indicated that he was employing an assembly of spring = washers=20 for installation of his composite over wood propeller He solicited my = comments=20 on this approach, and suggested that this would be a good basis for some = analysis and an informative article. Since I had previously of some = reference to=20 this practice, I agreed with him on both counts, and started searching = out=20 reference materials

It is always a source of amazement for me, how = things which=20 are very simple on the surface, have a very complex nature when examined = closely. Propeller bolts are just such an enigma. For a metal prop, = things are=20 virtually that simple. Just follow the usual bolt practice, and torque = them up=20 just short of yield, and the just keep them from backing out. For a wood (or primarily wood = core=20 composite) propeller, the =93wicket=94 gets stickier than the proverbial = tar=20 baby. The crushing = strength, and=20 =93crushing=94 modulus of most woods used are relatively modest, and the = usual bolt=20 torque tension loads would severely damage the wood structure. This is = further=20 aggravated by the dimensional changes of the wood as moisture is = absorbed and=20 released.

PROPELLER LOADS

Just how tightly do we have to secure a propeller? = What are=20 the forces and loads that are trying to take the prop off your airplane? = The=20 first thing you think of is the thrust forces that are pulling (or = pushing) the=20 plane around the sky. These loads are the least of our problems. = Another, more=20 troubling load is the gyroscopic precession as the plane direction is = changed by=20 pitch and yaw. With the lighter weight wood props, this is seldom a = serious=20 problem. However a big metal constant speed prop can react a load of as = much as=20 600 ft lbs with a yaw rate of one radian per second. This can be a serious problem = if one=20 engages in violent aerobatics. The fabled Lomchavok uses this precession = force=20 to turn the stalled airplane end over end (and broken crankshafts have = been know=20 to occur).

The big=20 need is for the clamping force, which acts much like the clutch disk in = a manual=20 transmission automobile. Although the classic prop hub has the drive = lugs, the=20 primary drive force is still this =93clutch=94 action. If it were not = for this=20 friction, the prop would cyclically slip back and forth in the hub. The=20 situation is further aggravated by the large displacement four cylinder = engines=20 typically used in aircraft. With only two power pulses per turn the peak = torque=20 values are higher than the rated steady values, and are actually = cyclically=20 reversed twice each turn. Once the shrinkage has reduced the preload, = the=20 cycling can induce alternating slippage at the flange face. The = resulting heat=20 further dries the wood, and a totally charred prop hub can result.

I have personally seen the result of just such a = scenario.=20 The Continental IO-240 has a small prop flange designed for metal = propellers,=20 aggravating this situation. On a flight to a local fly-in, engine = roughness was=20 noted as the destination approached. An expedited landing was initiated = without=20 problems, but as the engine was cut the propeller looseness could be = visually=20 seen. The hub of the wood prop was charred, and delaminated. A = replacement was=20 borrowed for the trip home, and the damaged prop now hangs on the wall = as a=20 visual reminder. This application normally employs a 4-inch prop = extension, and=20 an extension transitioning from the Continental hub to an S.A.E. number = 2 flange=20 was mandated for all subsequent installations.

The preload on a wood propeller must be = moderated to=20 avoid a crushing failure of the wood. The crushing strength of wood = varies with=20 species and density, ranging from about 1700 psi for maple down to about = 840 psi=20 for spruce. The rather = aptly named=20 =93crush plate=94 for most props has about 18 square inches in bearing. = Most of the=20 wood varieties selected for propellers are on the high end of this = range.=20 Staying a bit below the high end at a target value of 1000psi, this = would equate=20 to a total clamping force about just under 18,000 lbs, or about 3000 = pounds=20 force for each of the six bolts. As stated in a previous article on = preloading=20 bolts, and as a general truism, determining preload on a bolt using a = torque=20 wrench is a very inexact measure. You might at first think that this is = a=20 relatively simple treatment of the analogy to driving a force up the = inclined=20 ramp representing the pitch of the thread. Sorry! No cigar. The = component of the=20 effective ramp angle is so obscured by the other friction forces, that = it is=20 totally ignored in the usual prediction, As most of you are aware, the=20 coefficient of friction varies widely with surface finish, and degree of = lubrication, as well as the properties of the two materials in rubbing = contact.=20 The usual assumption in this case is smooth steel to steel, lightly = lubricated.=20 Lightly lubricated generally means that you wiped off any visible = liquid, but=20 did not clean with any degreaser, which is about what you would do to = avoid=20 rust. Torqueing a bolt involves at least two surfaces turning against = friction.=20 The thread , of course, and the washer face of the bolt. The thread = friction has=20 a multiplier because of the vee angle of the thread, which is a much = larger=20 driver than the lead angle of the thread. Lumping all these forces and=20 coefficients together for a 0.3 to 0.4 at the radius =93arm=94 at the = washer face of=20 the bolt gives us a pretty good WAG estimate. The attached table of = suggested=20 torque value for the different classes of bolts in automotive use, is = probably=20 targeting about 75 percent of the allowable yield strength in the thread = roots.=20 These would also be typical of the values used for metal props, but = would=20 vigorously crush a wood prop.

looking at recommendations in engine manuals and = propmakers,=20 we see torque values for the typical 3/8in prop bolt in wood props ,from = about=20 150 to 250 inch pounds. Using 200, and our above stated scheme that = would=20 be about = 200/(0.22r*.35f)=3D @2600=20 lbs (not to far from my straw man case above). In spite of the =93drive = lugs=20 (bosses) =93 on the SAE hubs, the primary drive forces, and especially = the torque=20 reversals from four cylinder engines, are taken in friction. The clamped = wooden=20 propeller acts very much like the friction clutch in your =93stick = shift=94. Sensenich uses birch for their = wood=20 props, and produce a high quality product with more experience in this = field=20 than almost anyone. One of their recommendations for torque is the = number of=20 degrees turned after initial =93bottoming=94. They are seeking a = compression of=20 0.006 inch per inch of hub thickness. The modulus of elasticity for = compression=20 perpendicular to the grain varies greatly with wood species, and even at = various=20 points within the same log. Working values for the birch used by = Sensenich would=20 appear to be between 200,000 and 300,000 psi/in/in. This would suggest a = preload=20 of roughly 1500 psi under the =93crush = plate=94 which is pretty close to the = crushing=20 limit stated for birch in the files of wood properties. With the beam = deflection=20 in the crush plate, the effective area under each bolt would be less = than 2=20 square inches, or a bit under 3000 lbf per bolt. Pressure on the = =93clutch plate=94=20 driving the prop would be on the order of 36000 pounds. With about a 0.4 = friction and a 2.5in radius this would drive about 3400 ft lbs of torque = without=20 slipping =96 well over the steady state value for most likely = engines,

However, the result of this situation is a spring = loaded=20 system, the primary spring being wood. A 0.006 in. per in shrinkage with = moisture change would completely relieve the spring force. Actually it = is even=20 worse than that, since wood will notoriously =93take a set=94 further = reducing this=20 spring load value

SPRING WASHERS

As mentioned earlier, Paul was installing his = prop with=20 spring (Belleville) washers. This is an approach that I had heard of = being=20 applied to wood props on the McCulloch drone engines used on = gyrocopters, and=20 one that I had often thought was a good idea for retaining clamping = forces in=20 use. The spring washers can provide high spring forces in a very compact = package, and with a variety of stacking techniques, provide tailored=20 combinations of force and deflection. They also provide visual = monitoring of the=20 preload on the bolt. The spring washers he chose, were from McMaster Carr (a well = known=20 supplier of industrial hardware). Two p/n =20 9712k32 . and one 9712K29 washers were stacked in parallel under = each=20 bolt. In parallel, the spring forces are additive. The specifications of = these=20 washers are:

=20 Gardner# =20 I.D. =20 O.D. = height thick deflection load flat load McMstr#

=20 1187-105 = 0.406 1.188 0.125 0.105 0.016 1950 2812 9712K32

=20 1000-105 = 0.406 1.00 0.118 0.105 0.010 1830 2657 9712K29

With 25 ft-lb torque (300 in-lb), the washers were = not=20 =93flat=94, which is as it should be. The specified total load for flat = should be=20 almost 8300 pounds, and the bolt load for 300 in-lbs torque would be = expected to=20 be about 3600 pounds, or bit less than half the bottoming load. and even = a bit=20 short of the 5700 rated displacement load. It would seem that this stack = is a=20 bit on the =93stiff=94 side. A tailored stack of washers in combinations = of parallel=20 and series arrangement would provide the desired preload with greater = deflection=20 possible without losing too much preload. A selection of hardened flat = washers=20 should also be included to protect the softer aluminum =93crush=94 = plate.

Checking a recent catalog from = McMasters-Carr, I=20 ordered an assortment of the spring washers with ID values suitable for = 3/8 inch=20 (typical propeller bolt size) bolts. I think That I will be excused from = any=20 copyright usage I printing the selected portion of the McMasters- = Carr cataloge showing the = variety of=20 possible choices for our = 3/8 inch=20 bolts. If you are using 7/16 inch or =BD inche bolts there are a similar = number of=20 choices 0n the subsequent page. Prices, are of course, subject to = change, but=20 are quite reasonable for the potential benefits derived.

Specifications of typical spring washers are:

ID =20 OD = Thick height deflect load flat load qty part# price qty part#ss price

The units selected for testing =96 shown to scale = on a 3.\/8=20 inch bolt

And the force/ deflection characteristics = are=94

The spring washers may be =93stacked=94 in various = combinations=20 to match the force and deflection characteristics that may be desired. = Stacking=20 in parallel =3D like Dixie cups. Will increase the loads for a given = deflection.=20 The forces are directly additive for this stacking arrangement. To = provide more=20 working travel for a given force change, a series stack can be used = (point to=20 point or =93flare=94 to flare) The series stack tends to be a bit = unstable, with the=20 points and flares slipping out of alignment,O>C>Baker one of our = KIS=20 bulders suggested a larger washer inserted at the flare to flar = intersection to=20 stabilize the assembly (attached figures show this arrangement'. The = force to=20 flatten the discs is the same, but the travel to get that load is now = doubled.=20

Two washers in series with the central stabilizing=20 washer.

A four Belville stack combining series and parallel = stacking.=20

With combinations of assemblies, a great number of = force to=20 displacement =91curves=94 can be provided. For our purposes, we decided = that a=20 fairly large displacement, with a rather =94flat=94 force curve would be = desired.=20 What this would mean is that the wood could change dimensions (shrink or = swell)=20 over a fairly wide range without a large change in clamping force. For a = birch=20 prop with 3/8 in bolts, the characteristics of four number 428 washers = in a series/parallel setup appeared = the most=20 promising. The total = assembly would=20 probably consist of a flat steel washer against the aluminum, the first = two=20 bellvilles point down, the second two points up, and a flat washer under = the=20 bolt head. This system appears to be nearly flat with roughly 200 inch = pounds of=20 torque, and close to 3000 pounds of clamping force for each bolt. Total=20 deflection would be over .050 inch, and even a 010 change in shrinkage = would=20 result in little loss in preload, =20 Different bolt diameters, and different wood hardness values = would=20 probably favor a different selection.

The local bearing forces of the sharp edges of the = Bellville=20 washers dictate the use of steel washers to protect the aluminum = =93crush plate=94,=20 and probably also the washer face on the associated bolt head or nut. = This total=20 =93stack=94 would add fairly measurably to the selected bolt length for = your=20 installation.

These curves were generated using wrench angle as = an=20 indication of inches of compression. and reading wench torque value at = each=20 point. This is admiditly a bit crude, but the direction and rough = magnitude can=20 be seen. The four Belville stack of two 423 Belvilles in parallel on = each side=20 of the stabilizing washer shows a great travel range with low fall of in = clamping force.

The result of such an installation would appear to = be clearly=20 a win, win situation. Weight and cost is quite minimal, and there do not = appear=20 to be any significant failure modes that have been increased by this = system.=20 Comments and suggestions are solicited.

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