How are RENCOL^® Tolerance Rings designed?

How a tolerance ring works is fairly simple, but in practice the design and specification of tolerance rings is much more involved. In a simple application such as RENCOL^® Bearing Mount, it can be relatively easy to specify a tolerance ring. However, in more challenging applications or conditions, they are normally designed in collaboration with the user. For a tolerance ring to work, it’s important to specify the other two components correctly.

The RENCOL^® Tolerance Ring design process in these cases is a collaborative one at an engineer-to-engineer level.  For best results, the design should be initiated as early on in the overall system design stage as possible, as often customers need to adapt their components to fit a tolerance ring in (see the section below). The performance that a system using a tolerance ring can generate is dependent upon the following three elements:
•    Inner (male) mating component
•    Outer (female) mating component
•    Tolerance ring

As you move away from the early stages of the design process, and elements of the system are locked down, the flexibility and ease of introducing a tolerance ring into the system become more complex, as the flexibility to change key parameters are reduced.

Design Process Overview

The typical design process stages for are:
•    Capture and review system requirements – Detail of the type of information needed at this stage is outlined in the following section and covers both the performance requirements and information on the components the tolerance ring will be mating. This can be done with our RENCOL^® engineers on a conference call, via a project information request or through one of our local, technical sales experts.
•    Design the tolerance ring – This stage often involves using sophisticated design tools such as FEA to predict the performance of the tolerance ring and to make sure it achieves the performance needed. A design proposal will be sent, detailing the design inputs and our recommendations for the tolerance ring.
•    Prototypes – are manufactured on small scale machines to give performance indication.
•    Testing activities – To check and validate the performance testing at both components and system levels are often done. This can involve compression testing, shaker testing, torque testing and durability to name a few.  It’s important that the tolerance ring is validated in a system.
•    Industrialisation – Once the design has been validated in your system, we then industrialise the part. The level of industrialisation is dependent on your needs; from a simple submission all the way to automotive accredited APQP processes.

What needs to be considered when designing tolerance rings?

There are a few things need to be considered when designing a tolerance rings – some of which may not be immediately obvious if you have never had experience of using tolerance rings before.  The following are some items that we need to consider:

Changes to mating component dimensions:

There needs to be some space between the components (normally around 0.4-1.2mm radially) to fit the tolerance rings in.  Sometimes it’s not possible to change the dimensions (for instance the OD of a ball bearing to mount), and it is essential that the RENCOL^® Tolerance Ring designer knows this.

Component tolerances:

It helps to determine the overall range in radial tolerances that are seen and as a result the forces being applied. It is also needed to make sure the minimum (or sometimes maximum) forces are achieved in worse case conditions.

Maximum and minimum operating temperature:

This has the same effect as component tolerances (changes the force applied), if the components are different materials; so, thermal expansion also needs to be taken into consideration to make sure that always the right force is achieved.

Width available to mount the tolerance ring:

The width of the tolerance ring will affect how the waves are designed.  With a narrower tolerance ring, each wave will provide a smaller amount of radial force than if the tolerance ring was wider.  Knowing the space constraints is important to see if a tolerance ring can give the desired performance.

Radial loads applied to the system:

Loads being applied to the system are very important when designing a tolerance ring and getting it wrong can lead to failure.  The types of information that is needed are:
•    Nature of the load – Is the load constantly applied to the system, at the same location, in all operating conditions? Such a scenario could point load a static tolerance ring, or apply a cyclic load to a rotating tolerance ring.
•    Axial / In-line or Radial – What orientation is the load applied to the system and is it always present? Excessive axial loading could, for example, result in relative movement of the components.
•    Component tilt from loading – Does the externally applied loading result in misalignment, or tilt, of the components when assembled? Such loading types can alter the relative compression of the tolerance ring waves around its circumference and affect the performance of the system.

Are there any special considerations for torque transfer or slip clutches?

These applications tend to be more challenging and additional information are required.

TORQUE TRANSFER

•    How much torque is needed to transfer? – It’s important here to also specify the safety factor that you would like to achieve.
•    Radial Loads – Are there significant radial loads being applied? If there are there may need to be features applied to the components, such as a steps, to protect the RENCOL^® Tolerance Rings.

SLIP CLUTCH

In such systems the requirement is that the components move relative to each other when a certain torque value is exceeded. This is to provide protection to other components or external users.
•    Torque slip – What is the torque at which you want to slip the system by?
•    Torque slip variation – How much tolerance do you allow in the torque variation? This is normally expressed as a percentage of the nominal torque (such as ±15%) or can be expressed as actual values (such as ±5Nm). 
•    The amount of slip cycles required – The design of the tolerance ring and the components needed will vary greatly depending on the number of cycles. It’s also important to define your slip cycle: How many degrees of rotation? How fast is the movement? Does is reciprocate or is it in just one direction?
•    The type & specification of grease to be used – The composition of the grease has a big impact on the amount of friction between the ID of the tolerance ring and the shaft during torque slip events.

Do I need features on my components to fit a tolerance ring in?

The features needed depend on the requirements and normally it is down to the balance between how much radial load the tolerance ring has to withstand and the strength of the components.  Below are the 3 most common arrangements of tolerance rings with their benefits and drawbacks.

Free Arrangement

This is the most cost effective component arrangement as no additional locating features are required on the mating components. The challenge with this arrangement is that the tolerance ring will take the full extent of any radial loading applied to the system, and sometimes may need tooling to keep the tolerance ring in place while assembling.