Summary info on Disc Spring Manufacture

We thought we'd communciate the manufacturing process so that at least you can appreciate the lead times on any bespoke/custom disc spring order and also for you to appreciate the high cost of manufacturing these items.

Materials

With a few exceptions nearly all available tables and data sets are calculated with values based on a Young’s modulus ‘E’ of 206000 N/mm2 with μ = 0.3 and are therefore only valid for Disc Springs from spring steel to DIN EN 10132-4 and DIN 17221 (e.g. 51 CrV 4 or C 67S, SAE 6150). The use of other materials necessitates recalculation with correct value for Young’s modulus ‘E’. The material will come in plate form with material certificates. We are ordering excess material so that any rework is not delayed by having to source material again. The material is initially sandblasted to remove scale and to allow final surface preparation on surface grinders to ensure uniform thickness for workshop manufacturing processes. The quality management plan, as detailed in the online tracking tool, demands the first client QA hold-point here. This includes Material Test Certs for Chemical Composition, Young’s modulus, and material uniformity. Tolerance for thickness, across all the disc spring types part of this project is +-0.1mm, and must be applied to t’ for the contact surface disc spring washers.

Alternative Materials

The vast majority of our orrder are for dsic spring working in non-corrosive, low ambient temperature environments. In those instances where the environment so demands, appropriate mmaterial is used:

• Where the operating temperature is high, ie greater than 100 degrees centigrage, we generally use Inconel X-750
• Where the operating envirnoment is corrosive and a non-magnetic material is needed, we use Stainless Steel 301

Blank Profiling

We will have the various plates laser cut, into blanks with Dprofile > De. This allows for more efficient nesting and better yields. For large engineering projects, with demanding timelines, we plan at least a 10% excess in required material as a risk mitigation strategy against rework or any quality assurance failures.

Design

As part of our ISO9001:2015 Quality Management System, we generate a Disc Spring Design with all the necessary properties that you can expect from both the individual disc spring as well as the stack in whatever configuration you will be using it, Here's an example.. Our results may differ from most other manufactures because we use an updated method for calaculating the contact surface/annulus dimension. This is extremely important for disc springs with De greater than 150mm. Even more importantly, especially where disc springs are used in a stack configuartion, we acocunt for friction in our designs.

Ferrari vs Standard Method

Anyone who has designed a disc spring using contact flats from first principles will have stumbled onto the contradictions embedded in the DIN2092 standard. Ferrari has a nice paper on this which we recommend any design engineer working with ;arger disc spring stacks to read The Ferrari Method. There are large erros in the standard method when not considering friction and then not dealing with the inherent contradictions embedded in the DIN 2092 requirements where:

• At 0.75.h (the free height of the unreduced spring without annuli/contact flats), the load for a reduced spring must be equal to that of the unreduced spring, which is incompatible with the requirement that:
• The width of the contact flats should be approximately 1/150 of the outside diameter and the t’/t ratio of 0.94

So, if you look at the range from Mubea, Schnor et al, their larger disc springs with contact flats do not conform to the technical requirements of a disc spring. This becomes evident in more demanding heavier load environments, is compounded by dynamic loads and is an obvious issue for stacks.

This in our opinion is a serious issue, and where large disc springs are used in mission critical applications, such as Breaking Mechanisms on mining head gear, this is a critical consideration, as lives depend on it. This is why we provide a detailed design document to our customers, that requires sign-off, as well as independent 3rd party Load Characteristic Testing Certs for stack applications - nobody else provides this level of manufacture excel lence.

For more on updated design calcualtion methods, please visit our design page. The Reliable Pressings Disc Spring Design Summary.

CNC Machining

The specific detail design for each disc spring's contact surfaces is part of the data pack. The design detail is further discussed on the section on Load Testing Plan. This stage includes the preparation surface grinding to ensure that t’ thickness is well within tolerance. The surface load bearing top & bottom flats are machined at the specified final cone angle, so that these are level once the forming is completed. All radii are added at this stage as well,

Final Forming

We use a slow hyraulic press to form the disc spring to the required heigh.

Heat Treatment

Our Heat Treatment supplier is barely 2kms away from us, and they provide an excellent Hardening service to us. We will request through hardening to Rockwell C - 45 with a tempering stage to ensure the correct Bainite and Pearlite microstructures for toughness. Hardness certificates will be available.

Scragging and Pre-stressing

After heat treatment each spring is flattened at least once, and this reduces the overall height by means of plastic deformation. This avoids later plastic deformation during later loading.

Shot-preening

Shot peening can be very beneficial to springs subjected to dynamic loads, however surface bonding by means of shot peening is not recommended for springs carrying static loads.

Surface Treatment

We have two main surface treatments which we apply. The Geomet standard is often requested, and we have an adequate substitute for this. We do get requests for zinc plating but this is not advisable, and we do no provide any warranty on zic plated disc springs

• The standard and ubiquitous zinc-phosphate and oiling protection - This is the standard process generally applied to all low alloy steels unless otherwise requested, as in this instance. A zinc phosphate layer is produced on the surface, which is then impregnated with corrosion-protection oil. This surface treatment process has the advantage of being well known and its effects on the various frictional components of stacks from string sets allows for better predictability & alignment between Theoretical/Calculated and Actual Load Characteristic Curves. The disc spring stacks are in some instances already pushing these limits, as every parallel packet count in series is at or over the 15 threshold.
• Geomet - Zinc Flake Dip-coat Silver – An organic binder filled with zinc flakes, additional aluminium powder and PTFE for improved corrosion resistance and lubricity between coated items. The process is as follows:

1. De-grease, Sandblast & Surface Prep – removal of contaminant oils/greases, roughening of surface also facilitates adhesion later.
2. Coating – This is a dip coat process, with items submerged in the coating, and then drained, followed by a second dip coating if necessary.
3. Aesthetics (Optional) – A final spray with the same product, but in a different colour, can provide a smoother uniform finish.
4. Baking Cycle – This is the curing step of the process, and includes inspection for proper bond adhesions and full curing. Handover to Packing & Dispatch completes the process.

Load Characterstic Testing

Where required, we either conduct our own Stack Load Characteristic Curve tests or have these conducted by an independent and certified 3rd party - SGS Metlab or the CSIR. This is an extra cost but in high performance designs is often an absolute necassity.

Stack Packaging

Our disc spring products are made by qualifuied engineers with the practical requirements of installation thought of. Each Stack will come in its own wooden pack, with a copy of all documentation packaged inside. The wooden packaging specification is: 25MM SOLID TIMBER BOXES - 2 WAY ENTRY TO CARRY 1 TON SPREAD WEIGHT. The Wooden containers have dimensions, L X B x H, so that B & H = Stack Diameter and L = Stack Length. The material is export compliant. The pictures below illustrate the quality of packaging. Labelling will be as directed by the customer to reduce confusion on-site.