Results 1 to 3 of 3

Thread: Mixing new and used donuts

  1. #1

    Mixing new and used donuts

    A comment was recently made about avoiding any pre-compression issues, and
    reducing the initial purchase expense, by simply mixing new and old gear
    cushions. I have been mentally debating whether to comment on that posting,
    out of concern that my input might come across as 'sour grapes'. I have
    decided to weigh in on it anyway.

    There are two key reasons not to take this approach. You can weigh them as
    you see fit.

    First, the IPC (and some maintenance instructions) caution against mixing
    donuts. While that is a legality you could easily ignore, they don't say
    'why'.

    The second reason, the 'Why', is the part that matters. This type of
    rubber compound does not get into the needed 'elasticity range' until it is
    loaded. Once loaded (pre-compressed or compressed in service), it actually
    becomes more 'jelly-like'. An uncompressed/unloaded cushion will seem very
    hard indeed, having almost no 'give' at all. If you mix new and old
    cushions, particularly in the retract size (thicker and larger in diameter),
    what happens is that the old donuts will take virtually all the compression
    forces. It will take a little while before they flatten and harden, and the
    loads get transferred to the newer cushions, therefore 'softening them up' a
    bit. But at that point you only have part of the intended cushioning
    available, since the older cushions have now flattened out; and you are
    faced with re-doing the job. You will probably also have the sides of the
    old cushions squeezed out and riding on the inside of the gear housing, due
    to the 'overload' being applied to them. In most cases, this will happen
    soon enough that the newest additions will be more closely matched to the
    ones that were recently installed, so the side effects won't be so
    pronounced; but it will still be visible to the knowing eye. Part of the
    cushion stack will have more 'swell' in it than the other.

    No need to ask me how I know this..
    _______________________________________________
    BAC-Mail mailing list
    BAC-Mail@beechaeroclub.org
    http://www.beechaeroclub.org/mailman/listinfo/bac-mail

  2. #2
    Initial Climb Initial Climb
    Join Date
    Apr 2005
    Location
    Between Apple Valley Ca and Cripple Creek Co
    Posts
    44

    Mixing new and used donuts

    I would think if anyone was going to go through the trouble and exspense of the labor and down time that all new would be a no brainer. Reminds me of the guy who changed out his clutch and reused the throw out bearing that looked good only to be tearing it all back down a couple of months latter.

    Richard Wolf

    >From: Mike Rellihan <rellihan@rellihan.com>
    >Date: Mon Feb 20 12:20:39 CST 2006
    >To: bac-mail@beechaeroclub.org, musketeermail@yahoogroups.com
    >Subject: [BAC-Mail] Mixing new and used donuts

    >A comment was recently made about avoiding any pre-compression issues, and
    >reducing the initial purchase expense, by simply mixing new and old gear
    >cushions. I have been mentally debating whether to comment on that posting,
    >out of concern that my input might come across as 'sour grapes'. I have
    >decided to weigh in on it anyway.
    >
    >There are two key reasons not to take this approach. You can weigh them as
    >you see fit.
    >
    >First, the IPC (and some maintenance instructions) caution against mixing
    >donuts. While that is a legality you could easily ignore, they don't say
    >'why'.
    >
    >The second reason, the 'Why', is the part that matters. This type of
    >rubber compound does not get into the needed 'elasticity range' until it is
    >loaded. Once loaded (pre-compressed or compressed in service), it actually
    >becomes more 'jelly-like'. An uncompressed/unloaded cushion will seem very
    >hard indeed, having almost no 'give' at all. If you mix new and old
    >cushions, particularly in the retract size (thicker and larger in diameter),
    >what happens is that the old donuts will take virtually all the compression
    >forces. It will take a little while before they flatten and harden, and the
    >loads get transferred to the newer cushions, therefore 'softening them up' a
    >bit. But at that point you only have part of the intended cushioning
    >available, since the older cushions have now flattened out; and you are
    >faced with re-doing the job. You will probably also have the sides of the
    >old cushions squeezed out and riding on the inside of the gear housing, due
    >to the 'overload' being applied to them. In most cases, this will happen
    >soon enough that the newest additions will be more closely matched to the
    >ones that were recently installed, so the side effects won't be so
    >pronounced; but it will still be visible to the knowing eye. Part of the
    >cushion stack will have more 'swell' in it than the other.
    >
    >No need to ask me how I know this..
    >_______________________________________________
    >BAC-Mail mailing list
    >BAC-Mail@beechaeroclub.org
    >http://www.beechaeroclub.org/mailman/listinfo/bac-mail

    _______________________________________________
    BAC-Mail mailing list
    BAC-Mail@beechaeroclub.org
    http://www.beechaeroclub.org/mailman/listinfo/bac-mail

  3. #3

    Mixing new and used donuts

    I tend to agree - 4 donuts * 3 sets saves ~$500-$650 depending on fixed
    or RG - versus the total cost of the repair with down time and labor to
    tear and replace, jo-bolts, etc.

    Mine were redone in 4/2001 by the previous owner (I'm guessing with
    Raytheon parts) - the logbook entry has the Beech -7 part #s and clearly
    states mains plus nose gear although doesn't mention the jo-bolts at all
    (although in general the A&P for the previous owner seemed to be pretty
    minimalist in his log entries unfortunately).

    Anybody have a feel for the in-use lifetime of these? I'm not talking
    max life - I'm more talking reasonable life with good cushion
    performance. Just trying to figure out if it's practical to score a set
    to put on the shelf. I figure 5 years on the shelf is practical - 10
    might not be...

    One thing I haven't done is get on my back with the flashlight to see
    how the current set is doing. Unfortunately I just don't have the
    scratch short term to help out after dropping $11k on avionics...

    Rubber compounds tend to have a good shelf life if properly stored and
    cared for (this generally means _not_ in your hangar - hangars usually
    have lousy temperature extremes) - mostly moderation in both temperature
    and humidity.
    --
    Mark


    > -----Original Message-----
    > From: bac-mail-bounces@beechaeroclub.org
    > [mailto:bac-mail-bounces@beechaeroclub.org] On Behalf Of
    > rwolfhse@verizon.net
    > Sent: Monday, February 20, 2006 12:50 PM
    > To: Mike Rellihan; bac-mail@beechaeroclub.org;
    > musketeermail@yahoogroups.com
    > Subject: Re: [BAC-Mail] Mixing new and used donuts
    >
    > I would think if anyone was going to go through the trouble
    > and exspense of the labor and down time that all new would be
    > a no brainer. Reminds me of the guy who changed out his
    > clutch and reused the throw out bearing that looked good only
    > to be tearing it all back down a couple of months latter.
    >
    > Richard Wolf
    >
    > >From: Mike Rellihan <rellihan@rellihan.com>
    > >Date: Mon Feb 20 12:20:39 CST 2006
    > >To: bac-mail@beechaeroclub.org, musketeermail@yahoogroups.com
    > >Subject: [BAC-Mail] Mixing new and used donuts
    >
    > >A comment was recently made about avoiding any
    > pre-compression issues,
    > >and reducing the initial purchase expense, by simply mixing
    > new and old
    > >gear cushions. I have been mentally debating whether to comment on
    > >that posting, out of concern that my input might come across
    > as 'sour
    > >grapes'. I have decided to weigh in on it anyway.
    > >
    > >There are two key reasons not to take this approach. You can weigh
    > >them as you see fit.
    > >
    > >First, the IPC (and some maintenance instructions) caution against
    > >mixing donuts. While that is a legality you could easily
    > ignore, they
    > >don't say 'why'.
    > >
    > >The second reason, the 'Why', is the part that matters.
    > This type of
    > >rubber compound does not get into the needed 'elasticity
    > range' until
    > >it is loaded. Once loaded (pre-compressed or compressed in
    > service),
    > >it actually becomes more 'jelly-like'. An uncompressed/unloaded
    > >cushion will seem very hard indeed, having almost no 'give'
    > at all. If
    > >you mix new and old cushions, particularly in the retract
    > size (thicker
    > >and larger in diameter), what happens is that the old donuts
    > will take
    > >virtually all the compression forces. It will take a little while
    > >before they flatten and harden, and the loads get transferred to the
    > >newer cushions, therefore 'softening them up' a bit. But at
    > that point
    > >you only have part of the intended cushioning available, since the
    > >older cushions have now flattened out; and you are faced
    > with re-doing
    > >the job. You will probably also have the sides of the old cushions
    > >squeezed out and riding on the inside of the gear housing,
    > due to the
    > >'overload' being applied to them. In most cases, this will
    > happen soon
    > >enough that the newest additions will be more closely matched to the
    > >ones that were recently installed, so the side effects won't be so
    > >pronounced; but it will still be visible to the knowing eye.
    > Part of the cushion stack will have more 'swell' in it than
    > the other.
    > >
    > >No need to ask me how I know this..
    > >_______________________________________________
    > >BAC-Mail mailing list
    > >BAC-Mail@beechaeroclub.org
    > >http://www.beechaeroclub.org/mailman/listinfo/bac-mail
    >
    > _______________________________________________
    > BAC-Mail mailing list
    > BAC-Mail@beechaeroclub.org
    > http://www.beechaeroclub.org/mailman/listinfo/bac-mail
    >
    _______________________________________________
    BAC-Mail mailing list
    BAC-Mail@beechaeroclub.org
    http://www.beechaeroclub.org/mailman/listinfo/bac-mail

Similar Threads

  1. Mixing new and used donuts
    By Rellihan in forum Musketeer-Mail Archive
    Replies: 0
    Last Post: 02-20-2006, 01:19 PM
  2. donuts to go
    By n76lima in forum Musketeer-Mail Archive
    Replies: 0
    Last Post: 12-04-2005, 10:29 AM
  3. Donuts !
    By mperry07 in forum Musketeer-Mail Archive
    Replies: 1
    Last Post: 10-06-2005, 07:25 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Single Sign On provided by vBSSO