Month: October 2024

Toro MX4250 spindle maintenance

The Toro MX4250 is a ZT ride on mower (riding mower).

I previously reported that after 200h, the original spindle bearings were in very poor condition, about 3mm of play at the blade ends, and they were replaced. There are lots of Chinese parts on this “made in the US made” mower, so the bearings might well have been Chinese in origin.

The original spindles are not greaseable, and were fitted with 6203RS (rubber seals on both sides) ball bearings.

There is lots of stuff online about mower spindles, they are a significant maintenance problem.

A grab bag of online expert advice:

  • No need for greasing, the bearings are sealed for life (that might be less than 100h).
  • Modify residential spindles by adding a grease nipple like commercial mowers use.
  • RS bearings cannot be greased, the seal will pop out.
  • Punch the bearings out, remove the inner seals and put them back.
  • Fit a grease nipple and remove the rubber seal from the inside of both bearings.
  • Some commercial mowers are fitted with a grease nipple and metal shielded (ZZ) bearings.

Since these spindles hold a lot of grease, I intend greasing with a pneumatic greaser, and they tend to inject very quickly and the risk of an RS seal popping should be mitigated.

Above is a 6203ZZ (metal shielded) bearing. There is a gap of about 0.5+mm between bearing inner and the shield, shields are on both sides. Continue reading Toro MX4250 spindle maintenance

Last update: 6th October, 2024, 11:54 AM

Transformers and magnetic saturation

It seems that even a basic but sound understanding of transformers challenges lots of hams, and even online experts that have been heard to brag of their qualifications so as to intimidate others who might question their words.

So at ARRL EFHW (hfkits.com) antenna kit transformer – revised design #1 – part 2 I estimated that at a current of 4Arms marked the onset of non-linear B-H response, ie the onset of saturation.

One online expert proposed a method that would rate this transformer at maximum 4^2*50=800W at which magnetic saturation would occur.

The referenced article estimated saturation at more like 17000W.

Some very basic transformer concepts

Let’s talk about some really basic transformer concepts.

The diagram above from Wikepedia shows a rectangular magnetic core with two windings, a primary and secondary on opposite limbs of the core.

Note the phase polarity markings (+ / -) and the direction of (conventional) alternating current. Continue reading Transformers and magnetic saturation

Last update: 5th October, 2024, 5:04 PM

ARRL EFHW (hfkits.com) antenna kit transformer – revised design #1 – part 2

This article continues on from several articles that discussed the ARRL EFHW kit transformer, apparently made by hfkits.com, and the revised design at ARRL EFHW (hfkits.com) antenna kit transformer – revised design #1 – part 1.

This article presents a saturation calculation.

You will not often see saturation calcs (for reasons that will become apparent), though you will hear uninformed discussion promoting FUD (fear, uncertainty and doubt).

Lets assume that the core is capable of maximum continuous power dissipation of 10W (limited by factors like safe enclosure temperature, human safety, Curie point etc).

Now let’s estimate the magnetising current for 10W of core dissipation with 3t primary

Starting with the expected permeability above… Continue reading ARRL EFHW (hfkits.com) antenna kit transformer – revised design #1 – part 2

Last update: 4th October, 2024, 6:01 AM

Fact check: “For an antenna, if it doesn’t resonate, it really doesn’t radiate!”

An example of the utter nonsense posted on social media.

My very first posting as a trainee was to Bringelly HF receiving station in 1970. It had Rhombic antennas every 30° of the compass, and a few other antennas, but the mainstay of operation was the set of Rhombics.

The nearby transmitting station at Doonside had a similar antenna arrangement of Rhombics fed with two or four wire open transmission lines to transmitters in a central building, for most operations, no coax involved between transmitters up to 30kW and antenna feed points. Continue reading Fact check: “For an antenna, if it doesn’t resonate, it really doesn’t radiate!”

Last update: 1st October, 2024, 5:46 AM