TechArchive

Original article date: March 1998

Trasmital Bonfiglioli has made a strong push into the UK planetary gearbox market. UK sales manager Russell Cleasby takes another look at this option.

The planetary gearbox offers some interesting advantages and can be proposed as a good alternative to parallel shaft gearboxes in various industrial applications. The advantages are more marked in applications requiring gearboxes with:

• high reduction ratios
• high transmissible torque
• high radial loads on output shafts

In planetary gearboxes each gear has several teeth in contact at once whereas in spur gears only one tooth works at a time. When transmitting the same torque this means that the stresses on each loop are lower so given the same load the planetary needs much smaller gears. Smaller dimensions given the same rotation speed will mean lower peripheral speed on the pitch diameter so spur gears used in a planetary system will have similar noise levels to helical gears used in a parallel shaft system.

The forces transmitted through the gears in a planetary reduction system are balanced. Their resulting force is nil only the torque being transmitted to the output shaft through the planetary carrier. As a result in a planetary gearbox the bearings supporting input and output shafts are only taking the loads acting on the shafts from outside the gearbox. Conversely in a parallel shaft system the resulting internal and external forces must be taken up by the bearings that support the gears. Because of this a planetary gearbox can withstand higher radial loads on the shaft.

Gearbox Options: Features and benefits

The Daily Planet

I’ve done a lot of work on power losses in gearing and gearboxes. The Lohmann + Stolterfoht article (Jan 98) didn’t accord with my experience. Planetary gearing is potentially LESS efficient than parallel shaft. The losses in any gearbox are an equation with three main factors:

- contact losses between mating pairs of gears:

(usually very low indeed; for example 0.1% for a good spur gear pair 0.5% for hypoid 0.8% for a worm pair)

- viscous losses from squeezing the oil film sliding losses etc in hypoid and worm:

(less than 2% at the worst and very small for spur pairs)

- churning losses from throwing the oil around in the casing:

(this is the big one; in a tight casing with a high oil level this can lose 40% at high speed; it tends to be a fourth power relationship with pitch line velocity and is the main loss-maker which is why aircraft and any racing applications always dry sump their gear casings and squirt oil into the gear pair meshing point as it comes out of mesh.)

The real experts on this are the guys who design helicopter power transmissions e.g. at Westland. Iearnt a great deal from one Brian Shotter who was their man in the 70s (now retired). You might persuade the top gear man there (or Shotter in his retirement) to contribute you an article. It might shame the industrial gearbox engineers into improving their designs – most are absolutely dreadful and there really is no excuse for turning out a box with more than 5% losses in almost any circumstances!

Dr Colin Mynott
Product Development Consultant

Paul Sixsmith L+S comments:

Really on a well engineered drive the seal drag is the biggest loss. I was really making the comparison with worm boxes which is less than 50% efficient. A planetary gearbox is comparable in cost to a worm box and comparable in efficiency to a parallel shaft box. Even a two-stage planetary gearbox is likely to be more than 95% efficient. Having said that the planetary gearbox is no more efficient than any box of similar type but a parallel shaft gearbox every bit as efficient is three times as big and is not as easy to use.

Looking at the helicopter gearbox Colin Mynott is absolutely right. You could have the performance of a helicopter gearbox in general engineering if you are prepared to pay helicopter prices! Try persuading somebody to put an oil circulation system on a gearbox! No way it’s added complication. Forced lubrication is an absolute last resort. I think we are doing a good job in mainstream industry within a budget.

Russell Cleasby Bonfiglioli comments:

When we talk about any comparisons between gearbox types we must first understand when to use planetary gearboxes. Sensible applications start with input powers of 2.2kW and upwards with output shaft speeds of 20rpm or slower. In addition you can look at any application requiring high transmittable torque high radial loads on the output shaft or high transmission loads.

Within these parameters when compared with work gearboxes savings are made on gearbox efficiency and when compared with worm in-line or parallel shaft gearboxes savings are made on volume weight and unit cost. This is because the compact design and high power capacities of the planetary gearbox offers the possibility of simpler and less expensive installations.

As to improving the designs of industrial gearboxes a balance has to be struck between performance manufacturing costs and maintenance cost. It’s like comparing the family car gearbox with one from an F1 racing car. But advances in manufacturing technology and design capabilities are improving the industrial gearbox. Twenty years ago the design characteristics of planetary gearboxes were not thought of as cost-effective but they are today.

Cost comparisons

Without specifically mentioning names here are some Bonfiglioli price comparisons of planetary gears against commercially available alternative gear systems. The comparisons are based on the following conditions:

Service factor of no less than 1.2

Duty cycle of 8 hours/day

Continuous running with a uniform load

Standard 4-pole motors to IP55

We couldn’t let this go without asking some competitors for their views!

The planetary unit is a good unit but if it is as good and as price competitive as they claim why are Renold David Brown SEW Flender etc. not out of business?

It is interesting to examine the powers and speeds used in the comparison. All of them are very low rpm (10 2.4 and 7.5). A survey carried out a year or so ago showed that 80% of all power transmission drives used by OEM customers were between 130 and 50 rpm and only 5% of drives were below 10 rpm. If this supplier wishes to aim for this market sector then it is aiming at a small one!

Also the prices used appear to me to be list prices shown against nett prices. If we were to use our normal OEM discount then the non-planetary prices would be about correct based on Renold products.

Trevor Priestley
Renold Engineering Products

Simply there is always a less expensive solution to a particular drive requirement but the customer or space available or configuration dictates what has to be offered. If we get an enquiry that doesn’t stipulate the configuration in any way we offer the most economical solution. Given a free rein to offer any type of gearbox to do the jobs in question we could probably beat all the prices quoted by Bonfiglioli by using something different than what is actually quoted in the comparisons made.

In the first of these examples (7.5kW at 10 rpm) our most economical solution would have been an in-line coaxial geared motor with helical gears which would actually have been less expensive than the planetary gearbox would be.

The double reduction gearbox is becoming a thing of the past and would not be the most economical solution in this application. Here we could have offered a parallel gearbox for less than (UK pounds)3600. Certainly the best applications for planetary gearboxes are very high ratio and very low speed applications where you need a coaxial drive: where you have 2.4rpm coming down from 1500 rpm then that is the niche that that type of gearbox is in and where it is most competitive. They are a lot smaller. The down side is that they can very easily overheat because of their low mass so unless they are running very slow and continuously they tend to have thermal problems. Then you would have to use a larger planetary gearbox or use auxiliary cooling which would push up the price.

Another point is that there is no legislation that says we have to sell a gearbox at any price. Of course our prices also vary considerably with volume so are we comparing the prices at the same level?. If we wanted to we could give gearboxes away with (UK pounds)100 attached in an envelope so these sorts of comparison are very dangerous. The only fair comparisons are to do the comparisons against their own alternatives.

David Makin
Business Development Director
David Brown Radicon

• Bonfiglioli
• Russell Cleasby
• 1925 852667

March 1998