Cable Tension Design Guidelines / Literature

[Hydromechdude]

Hi,

I'm looking to calculate the required cable tension in a closed loop installation (two quadrants with cable wrapped around both). One of the quadrants is connected to a handle for activation, the other quadrant is connected to the rest of the system.

What are the guidelines for determining cable tension? Any reference literature I could review? I'm new to flight controls.

For interest, my cable is 3/32" diameter. My assumption is that I want to ensure I have enough tension to preclude cable sag under its own weight and for any temperatures changes. I'm not quite sure if the system has a tension regulator, but my I believe it does.

The current cable has about 70 lbf tension, and the length will be increasing from approximately 40 feet to about 50 feet (actual distance is about 20 feet, but I doubled due to the quadrants and one wire running fore and the other aft).

Thanks,
HMD

 

[rb1957]

if the wire wraps around the quadrants, then an important consideration for cable tension is friction ... sounds abit suspect to me.

i'd prefer to see the cables in to 1/2 loops, anchored at both ends to the quadrants, adjustable turnbuckles to keep things taunt. this way the control input puts one cable in tension, which pulls on the slave quadrant.

 

[Hydromechdude]

Sorry, suppose I wasn't clear. I'm not looking at the assembly exactly, but rather I'm only curious about what the tension should be on the cable.

I'm increasing the overall length of the assembly right now, and as a sanity check I want to determine the new cable tension required. With that said, I don't know how to determine what the cable tension required is. I know the old installation's tension because it's specified in the manuals.

Thanks.

 

[WKTaylor]

Hydromechdude... not so simple...aircraft/installation specific.
Per SAE AS94900 [was MIL-DTL-9490], para...

3.6.2.6.13 Cable Tension
Cable rig loads shall insure positive cable tension in control and return legs of closed-loop cable installations under all
operating conditions including airframe deflection and differential expansion and contraction between the cable and
airframe structure throughout the designed operating temperature range. The cable return leg may be allowed to go slack
when the control leg is loaded above the normal operating load at any load up to limit load. The slack shall not result in
snagging on any adjacent cables, equipment or structure and there shall be no hazardous loss of system performance.
Cable tension regulators shall be provided only if positive cable tension cannot be maintained in both legs, with
reasonable rigging loads.

NOTE. System static strain, operational-load strain and thermal expansion and contraction of the structure and the steel cable [aluminum has a much higher expansion/contraction rate than steel... while composites have very LOW expansion/contraction relative to steel] are all case specific. NOTE: longer cable runs MAY dictate stronger/stiffer cables.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[Hydromechdude]

Thanks Wil.

That's the impression I got from what I've been looking at. Going through the Boeing design manual right now for cables.

Question for you. If you had a brand new design and installation, how would you calculate the tension required in order to preclude sagging and snagging on something? Obviously this depends on # of pulleys, location of the pulleys, unsupported length, etc. Is there some kind of formula that takes this into account?

Thanks again.

HMD

 

[rb1957]

i guess maybe i wasn't clear either.

if you're wrapping the cable around the two quadrants, then friction between the cable and the quadrant is the only loadpath connecting teh two quadrants. I'd've thought that this would design the cable tension ... how much quadrant torque (from the input force) can the cable transfer without slipping ?

 

[WKTaylor]

Two other [just-as-vague] references for control cable design...

MIL-HDBK-1599A BEARINGS, CONTROL SYSTEM COMPONENTS, AND ASSOCIATED
HARDWARE USED IN THE DESIGN AND CONSTRUCTION OF AEROSPACE MECHANICAL SYSTEMS AND SUBSYSTEMS. Requirement 206: Control Cable System.

MIL-B-87146 BEARINGS, CABLES, AND PULLEYS GENERAL SPECIFICATION FOR. Fill-in-the-blank system is useful for total system design [cables, terminals, bearings, pulleys, etc].

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[Hydromechdude]

Thank you very much, rb and Wil.

I concluded that for a couple additional pulleys and about an eight foot increase in cable length (initial was probably 100+ feet), the change would not be very much. The cable can go up to 500 lbs load and we're no where near that.

I'll look through the MIL references when I get some time. My problem is that I'm still not sure how to calculate the actual tension required if I had a new design in front of me. I don't know whether the MIL references answers that, though. Could I get this information from general cable and pulley design manuals? Not necessary aerospace.

There's the stretch in the cable that needs to be accounted for when it's pulled in tension. That's fine. But then you have to account for sag in the cable due to its own weight. That I'm less certain of. How do you calculate that? Assume a point load in the middle of span, based on the weight of the cable in that span?

Just thinking out loud right now.

 

[WKTaylor]

From MMPDS-05 1 April 2010

8.3 BEARINGS, PULLEYS, AND WIRE ROPE

Bearings — Design, strengths, selection criteria, and other data for plain and antifriction bearings
are found in AFSC Design Handbook AFSC DH-2-1, Chapters 3 and 6.

Pulleys — Pulley strengths and design data are to be utilized in accordance with Specification MIL-P[DTL]-7034.

Wire Rope — Strengths and design data for wire rope are to be selected from the following specifications, whichever is appropriate: MIL-DTL-83420 or MIL-DTL-87161.

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.