Sprag clutch with disengagement

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Discussion Overview

The discussion centers around the functionality and design of sprag clutches, specifically the possibility of disengaging the overrunning clutch function when not under load. Participants explore theoretical and practical aspects of sprag clutches in the context of applications such as DIY electric bikes and small engines.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Conceptual clarification

Main Points Raised

  • One participant questions the existence of a sprag clutch that can disengage when not under load, noting that traditional designs do not offer this feature.
  • Another participant suggests looking at recoil starters on small engines as a potential example, but this is challenged as not being relevant to the original question.
  • There is a proposal that if control over engagement is introduced, it may no longer function as a sprag clutch, raising questions about the definition and functionality of such clutches.
  • A participant describes a specific application involving a DIY ebike, expressing the need for a sprag clutch that allows for free rolling when the motor is not powered, suggesting a mechanism to disengage the sprags.
  • Concerns are raised about the difficulty of modifying existing sprag clutches to include a disengagement feature without adding significant complexity or space requirements.
  • Some participants discuss the idea of using two sprag clutches in series opposition as an alternative, though this is met with skepticism regarding its practicality and space efficiency.
  • A link to a design for a releasable sprag clutch is shared, prompting further discussion about its compatibility with existing concepts.
  • There is a suggestion that a manually operated clutch or a mechanism that moves axially could be necessary to achieve the desired disengagement functionality.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of modifying sprag clutches for disengagement, with no consensus reached on a viable solution. The discussion remains unresolved regarding the practicality of existing designs and the potential for new mechanisms.

Contextual Notes

Participants note limitations in current sprag clutch designs, particularly regarding their inability to disengage under specific conditions. There are unresolved questions about the definitions and operational parameters of sprag clutches versus other types of clutches.

Stormer
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All types of sprag overrunning clutches have some sort of spring to keep the rollers or sprags in contact with the running surfaces so they can engage and wedge to lock the two surfaces together when turning in the locking direction. But is there any kind of sprag clutch where you can dissenguage the overrunning clutch function (when not under load)? Seams like a pretty easy thing to do, but i have not seen any sprag clutch made with this function.
 
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Look at the recoil starter (search the term) on a small engine.
 
jrmichler said:
Look at the recoil starter (search the term) on a small engine.
That is just a normal sprag clutch. Not what i am asking for.
 
If you take control over the engagement of a sprag clutch, it ceases to be a sprag clutch. Will you independently lock or free the directional sprag clutch?

It is normal to select between two shaft speeds using two friction or dog clutches. The complexity and cost can be reduced by replacing the slower clutch with a sprag clutch, that will also prevent a locked condition when both are engaged.
 
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Baluncore said:
If you take control over the engagement of a sprag clutch, it ceases to be a sprag clutch. Will you independently lock or free the directional sprag clutch?
My goal is to make my DIY ebike be able to roll backwards freely when the motor is not powered. On most crank / middrive ebikes there is a sprag clutch between the motor drive and the crank so one does not cause drag for the other. But this causes the motor to drag when trying to roll the bike backwards. So I want a sprag clutch that works like normally as long as the motor is powered, and that fully release in both directions when the motor is not powered via some solenoid or something like that to release the sprag tension.

Just having a ring or something like that to push against the sprags to back them up against the spring tension maybe 1mm would disengage them form the running surface and achieve this, but it is difficult to do in a normal sprag clutch that is not made to have this function. So i think it is strange that i can not find any sprag clutches that have this function built in.

I will not add a separate clutch to do this function because that would take up to much space.
 
Last edited:
Stormer said:
Just having a ring or something like that to push against the sprags to back them up against the spring tension maybe 1mm would disengage them form the running surface and achieve this, but it is difficult to do in a normal sprag clutch that is not made to have this function.
When engaged and transmitting torque, the clutch will be locked solid, and you will not be able to force a disengagement. You will need to unload the clutch first.

You could use two sprag clutches in series opposition, and lock one or the other, rather than unlock one sprag clutch.
 
Baluncore said:
When engaged and transmitting torque, the clutch will be locked solid, and you will not be able to force a disengagement. You will need to unload the clutch first.
Yes as I say in the opening post I only need it to disengage when it is unloaded. Because the bike is standing still when i am going to roll it back to get it out of my basement or off the transport i'm on.

Baluncore said:
You could use two sprag clutches in series opposition, and lock one or the other, rather than unlock one sprag clutch.
That would take up even more space than adding a additional clutch to disengage the sprag clutch. And it would serve no function because if I could unlock one sprag clutch that is all that is needed, you don't need to add another one.
 
That appears to be a Sprag clutch, combined with a dog clutch.
 
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Baluncore said:
That appears to be a Sprag clutch, combined with a dog clutch.
Yea, it does.

Ultimately, there needs to be either a manually operated clutch somewhere in the drivetrain, or any braking action within the motor/drivetrain be disabled.

The only possibility I see for a clutch is something that moves axially to disengage.
Think the starter on your car, or the coaster brake built into the rear hub of a single-speed bicycle, or the clutch on a manual-shift car.
 
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